refactor(payload): retire remaining .expect() sites and drop quarantine

[goxberry]

What does this PR do?

Closes out the lading_payload .expect() cleanup stack. The crate-root #![allow(clippy::expect_used)] quarantine is removed from lading_payload/src/lib.rs, and the remaining 17 production .expect() sites are addressed. lading_payload is now fully covered by the workspace-level clippy::expect_used = "deny".

Sites covered

13 sites → .unwrap_or_else(... unreachable!("...")) (structural infallibles)

File:line	Site	Why infallible
templated_json/generator.rs:186	NonZeroI64::new(new_ms)	Existing // the error branch is unreachable in practice comment documents the positivity invariant
templated_json/config.rs:288	serde_json::to_string(s) on a &str	serde_json cannot fail on a string
templated_json/config.rs:443	parts.last() after str::split	str::split always yields ≥1 part
dogstatsd/metric.rs:149	ZeroToOne::try_from(sample_rate) after clamp(0.0, 1.0)	Clamp guarantees range membership
dogstatsd/metric.rs:263	values.pop() after a known values.push(value) 100+ lines above	Vec has ≥1 element
syslog.rs:75-76	OffsetDateTime::from_unix_timestamp(base_ts + offset)	Range [2020, 2030] is well inside the valid OffsetDateTime range
syslog.rs:77	.format(&Rfc3339) of a valid OffsetDateTime	RFC-3339 formatting cannot fail on a constructed date
syslog.rs:87	serde_json::to_string(&rng.random::<Message>())	Message is a struct of primitive fields with derived Serialize
datadog_logs.rs:57	str_pool.of_size_range(rng, 1_u8..16)	Pool is constructed with size 1_000_000 (datadog_logs.rs:101); 1..16 always fits
datadog_logs.rs:60	serde_json::to_string(&rng.random::<Structured>())	Same shape as Message
fluent.rs:129, 135	str_pool.of_size_range(rng, 1_u8..16)	Same pool pattern
common/strings/random_string_pool.rs:189	pool.of_size(rng, sz)	Caller convention: pool is sized larger than length_range

4 sites → fn-level #[expect(clippy::expect_used, reason = "...")] (panic is the documented contract)

Function	Why the panic is intentional
dogstatsd/metric.rs::MetricGenerator::generate	self.templates is validated non-empty at construction; an empty templates vec here is a serious logic bug, as documented by the existing SAFETY: comment
dogstatsd/service_check.rs::ServiceCheckGenerator::generate	Same shape: self.names is validated non-empty at construction
dogstatsd/common.rs::NumValueGenerator::new	Uniform::new_inclusive(min, max) fails when min > max. ConfRange::Inclusive { min, max } does not validate min ≤ max at deserialization; a misconfigured config panics here with the existing diagnostic message

Quarantine removed

The #![allow(clippy::expect_used)] line is removed from lading_payload/src/lib.rs. From here on, any new .expect() in production code in this crate is a hard CI failure.

Motivation

Final PR in the lading_payload cleanup sub-stack. The stack started by #1882:

1. chore(clippy): forbid .expect() in production code #1882 — workspace lint + crate-root quarantines
2. chore(clippy): drop expect_used quarantine for lading_throttle #1883 — drop lading_throttle quarantine
3. refactor(payload): mark infallible .expect() sites as unreachable!() #1884 — Cat-3 (26 infallible sites)
4. refactor(payload): mark fallible-context infallibles as unreachable!() #1885 — Group A (5 ?-context infallibles)
5. refactor(payload): mark in-function invariants as unreachable!() #1890 — Group B (6 in-function invariants)
6. refactor(payload): annotate intentional-panic .expect() sites #1891 — Group C (11 fn-level #[expect] for handle contracts)
7. fix(payload): reject zero-byte arbitrary input for Block #1892 — Group D (real bug fix in Block::arbitrary)
8. refactor(payload): retire remaining .expect() sites and drop quarantine #1893 — Group E + drop lading_payload quarantine (this PR)

After this lands, the only crates still carrying a quarantine are lading_capture (149 sites) and lading (243 sites), which are addressed in later sub-stacks.

Related issues

Stacked on #1892.

Additional Notes

• cargo build --all-targets --all-features ✓
• bash ci/validate ✓ (full: shellcheck, fmt, check, custom lints, clippy, deny, config validation, nextest, loom, kani, buf)
• cargo clippy -p lading-payload --all-targets --all-features 2>&1 | grep -c expect_used → 0 (verifies the quarantine drop is safe)
• Diff: 13 files, +69 −42.

Note on parent PRs

While preparing this PR I noticed PRs #1885, #1890, #1891, and #1892 had fmt CI failures from earlier (cargo fmt --check whitespace) that I missed by running ci/clippy instead of ci/validate per branch. Those PRs have been force-pushed with formatting fixes. The changes are pure whitespace and don't touch semantics.

[goxberry]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• fix(lading): surface tooling/FUSE-mount panics as errors #1904
• fix(lading): surface remote-input parsing failures as errors #1903
• fix(lading): surface user-config parsing failures as Error variants #1902
• refactor(lading): drop expect_used quarantines and annotate bins #1901
• refactor(lading): annotate intentional-panic observer/target sites #1900
• refactor(lading): annotate intentional-panic network generators #1899
• refactor(lading): annotate intentional-panic file_gen sites #1898
• refactor(lading): annotate intentional-panic blackhole + small generator sites #1897
• refactor(lading): convert structural-infallible .expect() to unreachable!() #1896
• refactor(capture): retire .expect() sites and drop quarantines #1894
• refactor(payload): retire remaining .expect() sites and drop quarantine #1893 👈 (View in Graphite)
• fix(payload): reject zero-byte arbitrary input for Block #1892
• refactor(payload): annotate intentional-panic .expect() sites #1891
• refactor(payload): mark in-function invariants as unreachable!() #1890
• refactor(payload): mark fallible-context infallibles as unreachable!() #1885
• refactor(payload): mark infallible .expect() sites as unreachable!() #1884
• chore(clippy): drop expect_used quarantine for lading_throttle #1883
• chore(clippy): forbid .expect() in production code #1882
• main

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