fix: derive temperature unit from locale temperature preference, not distance system

[jamesarich]

What

Temperature display units were inferred from the distance measurement system (isFahrenheit = displayUnits == IMPERIAL). That is wrong for mixed-unit locales: ICU defines LocaleData.MeasurementSystem.UK as distinct from US precisely because the UK uses miles for road distance but Celsius for temperature. An en-GB phone correctly showed miles — and incorrectly showed °F.

How

• New TemperatureUnit expect/actual in core/common, deliberately decoupled from MeasurementSystem.
• Android actual delegates to androidx.core.text.util.LocalePreferences.getTemperatureUnit(), which resolves CLDR locale data and honors the Android 14+ Regional preferences temperature override (Settings → System → Languages → Regional preferences → Temperature), which was previously ignored.
• JVM (desktop) actual uses CLDR's Fahrenheit-default region list (US, BS, BZ, KY, PR, PW); the iOS stub returns Celsius like its existing measurement-system stub.
• Both derivation sites switched: CommonGetNodeDetailsUseCase and NodeListViewModel. Distance/speed continue to follow getSystemMeasurementSystem() unchanged.
• New Robolectric host test TemperatureUnitTest covers US (°F), UK (°C + imperial distance), Germany (°C), and the -u-mu-celsius regional-preference override.
• Updated docs/en/user/units-and-locale.md and docs/en/developer/measurement.md (with last_updated bumps) to document the decoupling.

Constitution Check (v1.3.3)

• I. KMP Core — new API is expect/actual; no java.*/android.* in commonMain. ✅
• II. Zero Lint Tolerance — spotlessApply spotlessCheck detekt pass locally. ✅
• III. Compose Multiplatform UI — no UI changes; composables consume the same booleans. ✅
• IV. Privacy First — no logging, no PII. ✅
• V. Design Standards — no visual changes. N/A
• VI. Documentation Freshness — both affected doc pages updated with last_updated: 2026-08-19. ✅
• VII. Verify Before Push — spotlessApply spotlessCheck detekt :core:common:testAndroidHostTest :core:common:jvmTest :feature:node:testAndroidHostTest :feature:node:jvmTest all pass locally; CI checked after push. ✅

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Summary by CodeRabbit

• New Features

  • Temperature displays now follow the system’s dedicated temperature preference, independently of distance units.
  • Supports Fahrenheit and Celsius preferences across locales and platforms.
  • Android regional temperature overrides are supported.

• Documentation

  • Updated user and developer guidance for temperature preferences, locale behavior, and mixed measurement systems.

• Tests

  • Added coverage for regional, locale-based, and Android temperature preferences.

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📝 Walkthrough

Walkthrough

The change adds a platform-specific temperature-unit API. Android, JVM, and iOS implementations provide the unit. Node temperature consumers use this API. Tests and documentation cover locale and regional preference behavior.

Changes

Temperature unit support

Layer / File(s)	Summary
Temperature unit contract and platform resolution core/common/src/commonMain/..., core/common/src/androidMain/..., core/common/src/jvmMain/..., core/common/src/iosMain/..., core/common/src/androidHostTest/...	Adds TemperatureUnit and getSystemTemperatureUnit(). Android reads regional preferences, JVM uses Fahrenheit locale regions, and iOS returns Celsius. Tests cover locale and regional override behavior.
Node temperature consumer integration feature/node/src/commonMain/...	Node details and node lists use the dedicated temperature unit instead of the distance measurement system.
Temperature unit documentation docs/en/developer/measurement.md, docs/en/user/units-and-locale.md	Documents independent temperature preferences, Android regional overrides, mixed-unit locales, and configuration steps.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Merge Risk: 🟡 Moderate · up to d2b59

Users who select Kelvin in Android regional preferences would still see temperatures in Celsius, so the change is not merge-ready until Kelvin is supported or the product explicitly decides to exclude that preference.

Sequence Diagram(s)

sequenceDiagram
  participant NodeListViewModel
  participant getSystemTemperatureUnit
  participant PlatformLocaleSettings
  NodeListViewModel->>getSystemTemperatureUnit: request temperature unit
  getSystemTemperatureUnit->>PlatformLocaleSettings: read locale or regional preference
  PlatformLocaleSettings-->>getSystemTemperatureUnit: return Fahrenheit or Celsius
  getSystemTemperatureUnit-->>NodeListViewModel: set Fahrenheit display mode

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🚥 Pre-merge checks | ✅ 6 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Tests Prove The Path, Not The End State	⚠️ Warning	TemperatureUnitTest calls getSystemTemperatureUnit() directly; reverting both changed node derivations to legacy measurement-system logic leaves all four tests passing, so no consumer path is proven.	Add consumer tests for en-GB and the override that assert MetricsState.isFahrenheit and NodesUiState.tempInFahrenheit; the legacy derivation must fail those tests.
Regression Coverage For Changed Behavior	⚠️ Warning	TemperatureUnitTest only tests the Android helper; no test covers CommonGetNodeDetailsUseCase.metricsState.isFahrenheit, NodeListViewModel.nodesUiState, or the JVM actual.	Add en-GB tests that assert Celsius with imperial distance through both downstream states, plus JVM tests for the Fahrenheit-region mapping; include the Android override at a downstream surface.

✅ Passed checks (6 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly summarizes the main change: deriving the temperature unit from locale temperature preferences instead of the distance measurement system.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Sibling Call Sites And Presence Semantics	✅ Passed	The PR changes locale temperature-unit selection only. It adds no scoped nullable/default-zero field or zero-guard change; NodeItem and NodeItemCompact already use nullable temperature presence che...
Moved Code Diffed Against Its Original	✅ Passed	The diff has no deleted or renamed declarations; it adds a separate TemperatureUnit API, preserves getSystemMeasurementSystem, and updates both former temperature callers to the new API.

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Actionable comments posted: 1

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Inline comments:
In
`@core/common/src/commonMain/kotlin/org/meshtastic/core/common/util/MeasurementSystem.kt`:
- Around line 33-36: Add KELVIN to TemperatureUnit and propagate it through the
Boolean temperature state, MetricFormatter.temperature(), and platform
implementations. Preserve regional Kelvin preferences by converting temperatures
to Kelvin and retaining the existing Celsius/Fahrenheit behavior; add or update
override tests covering Kelvin selection and conversion.

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📥 Commits

Reviewing files that changed from the base of the PR and between 582d320 and d2b593e.

📒 Files selected for processing (9)

• core/common/src/androidHostTest/kotlin/org/meshtastic/core/common/util/TemperatureUnitTest.kt
• core/common/src/androidMain/kotlin/org/meshtastic/core/common/util/LocaleUtils.android.kt
• core/common/src/commonMain/kotlin/org/meshtastic/core/common/util/MeasurementSystem.kt
• core/common/src/iosMain/kotlin/org/meshtastic/core/common/util/NoopStubs.kt
• core/common/src/jvmMain/kotlin/org/meshtastic/core/common/util/JvmPlatformUtils.kt
• docs/en/developer/measurement.md
• docs/en/user/units-and-locale.md
• feature/node/src/commonMain/kotlin/org/meshtastic/feature/node/domain/usecase/CommonGetNodeDetailsUseCase.kt
• feature/node/src/commonMain/kotlin/org/meshtastic/feature/node/list/NodeListViewModel.kt

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[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
7000	1	6999	0

View the full list of 1 ❄️ flaky test(s)

org.meshtastic.app.firmware.SoftDeviceQuirkCoverageTest::every nrf52840 model in the hardware catalog has a softdevice mapping

Flake rate in main: 75.00% (Passed 4 times, Failed 12 times)

Stack Traces | 0.376s run time

java.lang.AssertionError: nRF52840 models with no SoftDevice mapping: [136]. Factory erase is silently unavailable for these. Derive each from meshtastic/firmware boards/<board>.json build.arduino.ldscript (nrf52840_s140_v6.ld = 6.1.1, _v7.ld = 7.3.0), or add the hwModel to knownUnmapped with a reason.
	at org.junit.Assert.fail(Assert.java:89)
	at kotlin.test.junit.JUnitAsserter.fail(JUnitSupport.kt:56)
	at kotlin.test.Asserter.assertTrue(Assertions.kt:767)
	at kotlin.test.junit.JUnitAsserter.assertTrue(JUnitSupport.kt:30)
	at kotlin.test.Asserter.assertTrue(Assertions.kt:777)
	at kotlin.test.junit.JUnitAsserter.assertTrue(JUnitSupport.kt:30)
	at kotlin.test.AssertionsKt__AssertionsKt.assertTrue(Assertions.kt:44)
	at kotlin.test.AssertionsKt.assertTrue(Unknown Source)
	at org.meshtastic.app.firmware.SoftDeviceQuirkCoverageTest.every nrf52840 model in the hardware catalog has a softdevice mapping(SoftDeviceQuirkCoverageTest.kt:58)

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[jamesarich]

The shard-app failure is pre-existing, not from this change: the scheduled hardware-catalog update #6758 added the T-Echo-Card (hwModel 136) with no SoftDevice row, so SoftDeviceQuirkCoverageTest fails on every PR's merge with main (Codecov confirms a 100% failure rate on main). Fixed in #6776 — once that merges, a re-run of this PR's checks should go green. All other checks here pass, including shard-core, which runs the new TemperatureUnitTest.

🤖 Generated with Claude Code