refactor(capture): retire .expect() sites and drop quarantines

[goxberry]

What does this PR do?

Closes out the lading_capture .expect() cleanup. Both crate-root #![allow(clippy::expect_used)] quarantines are removed:

• lading_capture/src/lib.rs
• lading_capture/src/bin/fuzz_capture_harness.rs

11 production .expect() sites are addressed across the crate. Treatment mix mirrors the patterns from #1884/#1890/#1891/#1893: fn-level #[expect] for contract-style invariants where the .expect message is the contract, and site-level unreachable!() for structural infallibles.

Sites covered

7 fn-level #[expect(clippy::expect_used, reason = "...")] — the .expect() calls themselves are unchanged.

Function	Sites	Why the panic is intentional
test/writer.rs::InMemoryWriter::{get_bytes, get_string, parse_lines} + Write::write	4	InMemoryWriter is a test/fuzz helper (#![cfg(any(test, feature = "fuzz"))]). The mutex-poisoning and invalid-UTF-8 panics are documented contracts (see # Panics sections).
manager/state_machine.rs::StateMachine::record_captures	1	self.format.as_mut() — format is Some throughout the operating state of the state machine.
manager/state_machine.rs::StateMachine::drain_and_write	1	same
manager/state_machine.rs::StateMachine::write_metric_line	3	same (3 .expect() calls collapse to one fn-level annotation)
manager.rs::CaptureManager::start	1	self.shutdown.take() — shutdown is populated at construction and consumed exactly once here.

4 site-level .unwrap_or_else(... unreachable!("...")) — structural infallibles.

File:line	Site	Why infallible
manager.rs::RealClock::now_ms	(start_system_time + elapsed).duration_since(UNIX_EPOCH)	start_system_time is captured at program start in a modern epoch, well after UNIX_EPOCH.
accumulator.rs:230, 525	dogsketch.write_to_bytes()	prost::Message::write_to_bytes on an in-memory struct cannot fail.
bin/fuzz_capture_harness.rs:389	op_iter.next()	OpIterator::next() is structurally infinite — every match arm returns Some(...); the fallback is unreachable!().

Discovery note

The initial survey reported "149 production sites" in lading_capture. Once the quarantines were temporarily removed and cargo clippy -p lading-capture --all-targets --all-features was run, only 11 real sites surfaced. The bulk of the original 149 was inside #[cfg(test)] mod tests { ... } blocks at the bottom of .rs files — covered by allow-expect-in-tests = true once the crate-level allow is gone.

Motivation

Second-to-last sub-stack in the cleanup chain. After this, only lading (243 surveyed sites; similar pattern expected) remains to bring the whole workspace to zero expect_used violations.

Related issues

Stacked on #1893.

Additional Notes

• cargo build --all-targets --all-features ✓
• bash ci/validate ✓ (full suite, including kani proofs and loom tests)
• cargo clippy -p lading-capture --all-targets --all-features 2>&1 | grep -c expect_used → 0
• Diff: 7 files, +63 −15.

[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 👈 (View in Graphite)
• 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
• 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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