refactor(lading): convert structural-infallible .expect() to unreachable!()

[goxberry]

Summary

Mechanical Cat-3 conversions: replace .expect("...") with .unwrap_or_else(|| unreachable!("...")) (or |_| for Result) at sites where the surrounding code guarantees the invariant holds. No behavior change, no #[expect] annotations, no quarantine drops.

Examples of patterns converted:

• NonZeroU16::new(1).expect(...) on a literal nonzero constant
• "...".parse::<IpAddr>().expect(...) on a hard-coded valid string literal
• Response::builder().body(full(...)).expect(...) after a successful builder chain
• process tree/file_tree iterators known to be non-empty

This is the foundation of the stacked cleanup that eventually drops the four crate-level #![allow(clippy::expect_used)] quarantines on the lading crate.

Test plan

• `./ci/validate` passes
• CI green on this PR

[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 👈 (View in Graphite)
• 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
• 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

This stack of pull requests is managed by Graphite. Learn more about stacking.

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