refactor(payload): annotate intentional-panic .expect() sites

[goxberry]

What does this PR do?

Annotates 11 functions in lading_payload that contain intentional-panic .expect() calls with #[expect(clippy::expect_used, reason = "...")]. The .expect() calls themselves are unchanged — they document a contract and panicking is the intended response.

Annotated functions and the contract each preserves

Function	Why the panic is intentional
block::Cache::read_at	Doc comment already declares: "Function will panic if reads are larger than machine word bytes wide." The internal usize::try_from(u64) calls are that contract.
RandomStringPool::using_handle	Handle has two variants (PosAndLength, Index); only PosAndLength is valid here. A cross-variant handle is a cross-pool misuse — a programming error.
StringListPool::using_handle	Symmetric: only Index is valid here.
Display::fmt for Event (dogstatsd/event.rs), ServiceCheck (dogstatsd/service_check.rs), Count, Gauge, Timer, Dist, Set, Histogram (all in dogstatsd/metric.rs)	Tag keys/values are stored as opaque handles into self.pools.tag_name / self.pools.tag_value. The handles were issued by those exact pools at the metric/event/service-check construction site, so the lookup cannot fail unless an internal invariant has been violated.

Why #[expect] and not unreachable!() or panic!()

For these sites the .expect() message is the contract, and the panic is the intended behavior. Converting to unreachable!() would lose the documented contract text; converting to panic!("...") would just sidestep the lint while saying the same thing more loudly. #[expect(clippy::expect_used, reason = "...")] keeps the code untouched and records, at the source, why the lint allows it. If a future change makes the lint stop firing inside the function (e.g., all .expect() calls get removed), #[expect] itself warns and the annotation can be deleted.

The reason strings are tight per-site descriptions; they're not boilerplate.

Motivation

Fifth per-crate cleanup PR in the stack started by #1882. Remaining sub-stack:

• Group D (next): real bug fix at block.rs:123 — the arbitrary::Arbitrary impl for Block calls NonZeroU32::new(total_bytes).expect(...) where total_bytes = u32::arbitrary(u)? can legitimately be 0. Fix: convert to arbitrary::Error::IncorrectFormat so the fuzzer rejects zero-byte inputs.
• Final: Cat-2 (function-signature changes) for ~33 remaining sites + drop the lading_payload quarantine.

Related issues

Stacked on #1890.

Additional Notes

• cargo build --all-targets --all-features ✓
• cargo clippy --all-targets --all-features ✓
• cargo test -p lading-payload --lib ✓ (244 passed)
• Diff: 7 files, +52 −0 (only attribute additions).

[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
• fix(payload): reject zero-byte arbitrary input for Block #1892
• refactor(payload): annotate intentional-panic .expect() sites #1891 👈 (View in Graphite)
• 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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