refactor(lading): annotate intentional-panic blackhole + small generator sites

[goxberry]

Summary

Annotate intentional-panic .expect() sites in the blackhole subsystem and a few small generator constructors with fn-level #[expect(clippy::expect_used, reason = "...")]. Convert the Cat-3-eligible Response::builder().body() and HeaderValue::from_static sites inside otlp/http.rs to unreachable!(), matching the rest of the codebase.

Sites annotated:

• `blackhole/otlp/http.rs::new` — serde_json on default OTLP responses
• `blackhole/tcp_rr.rs::run` — `set_nonblocking` on bound listener
• `blackhole/tcp_rr.rs::create_listener` — socket-setup syscalls
• `blackhole/tcp_rr.rs::server_thread_main` — mio Poll/registry setup
• `generator/container.rs::spin` — `u32::try_from` on bounded index
• `generator/file_tree.rs::spin` — `iter().cycle()` over non-empty Vec

Stacked on #1896. Part of the broader push toward enforcing clippy::expect_used = "deny" workspace-wide.

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 👈 (View in Graphite)
• 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
• 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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