chore(radio): remove 20% deadband for low volume

[3djc]

This improves settings sound level by special function or sound settings by source. It reduces the dead band before any sound can be played (previously volume was set a 0 until at least -80%)

This also simplifies the mechanic when hardware needs a fixed volume limitation

Summary by CodeRabbit

• Bug Fixes

  • More immediate speaker volume updates by replacing hysteresis with a deadzone-based input filter.
  • Removed internal volume scaling so headphone volume matches the user-set level.
  • Lowered minimum volume threshold for improved low-volume audio fidelity.

• New Features

  • Added trace logging for volume changes to aid diagnostics.

• Chores

  • Introduced configurable/default max volume handling and standardized per-target max volume to 20 for affected transmitters.

[pfeerick]

@coderabbitai review please

[coderabbitai]

@pfeerick Sure, I'll review the changes in this PR now!

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: f4d725c6-aa29-4c1d-9fda-920a7655474f

📥 Commits

Reviewing files that changed from the base of the PR and between ea083b5 and 952abfa.

📒 Files selected for processing (1)

• radio/src/hal/audio_driver.h

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📝 Walkthrough

Walkthrough

Refactors speaker volume input from hysteresis to a deadzone-based mapping, adds per-target VOLUME_LEVEL_MAX overrides, lowers TAS2505 VOLUME_MIN_DB with added trace logging, and has the board driver pass unscaled volume to the TAS2505 driver.

Changes

Audio Volume Control Refactor

Layer / File(s)	Summary
Volume Configuration & Constants radio/src/hal/audio_driver.h, radio/src/targets/tx15/hal.h, radio/src/targets/tx16smk3/hal.h	Added guidance comment and made VOLUME_LEVEL_MAX/VOLUME_LEVEL_DEF conditional (#ifndef) so targets can override; tx15 and tx16smk3 now #undef/#define VOLUME_LEVEL_MAX 20. I2S DMA macros reformatted.
Core Volume Input Processing radio/src/edgetx.cpp	calcVolumeValue() replaced hysteresis-based, stateful update with a deadzone-based computation using VOLUME_SOURCE_DEADZONE; removed requiredSpeakerVolumeRawLast.
TAS2505 Driver Volume Control radio/src/drivers/tas2505.cpp	Lowered VOLUME_MIN_DB from 60 to 45 and added a TRACE log at start of tas2505_set_volume() showing volume and headphone_mode; clamping/register writes unchanged.
Audio Board Driver Integration radio/src/boards/rm-h750/audio_driver.cpp	Removed unused timers_driver.h include; audioSetVolume() now forwards the unscaled volume to tas2505_set_volume() (headphone detection unchanged).

sequenceDiagram
  participant Input as User/Input
  participant EdgeTX as edgetx::calcVolumeValue
  participant Board as rm-h750::audioSetVolume
  participant Driver as tas2505::tas2505_set_volume
  participant Chip as TAS2505_IC

  Input->>EdgeTX: centered input (1024 + getValue)
  EdgeTX->>EdgeTX: apply deadzone -> compute requiredSpeakerVolume
  EdgeTX->>Board: audioSetVolume(requiredSpeakerVolume)
  Board->>Driver: tas2505_set_volume(volume, audioHeadphoneDetect())
  Driver->>Chip: set digital volume / write registers

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~25 minutes

Poem

🐇 I hopped along the audio stream,
Found deadzones where the signals dream,
The driver hums with quieter dB,
Boards pass volumes true and free,
A tiny rabbit claps—hooray, sound gleams! 🎶

🚥 Pre-merge checks | ✅ 3 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is incomplete. It lacks the required 'Fixes #' section from the template and does not follow the standard 'Summary of changes:' structure.	Add 'Fixes #' issue reference and structure the description to match the template format with 'Summary of changes:' section.
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: removing the volume deadband that prevented sound at low levels.
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.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 3djc/improve-audio-volume

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