chore: drop 30 dead_code allows that no longer cover anything

[Lstarsky0]

What and why

Another #5587 slice, the "stale allows" part of the tail. I ran cargo check --workspace --all-targets with RUSTFLAGS="--force-warn dead_code", then the non-test and test targets on their own, and matched every allow(dead_code) against the warnings inside the item it covers. For these 30, rustc reports nothing in the build the attribute applies to, so they suppress nothing:

• palette: adapt.rs (5), contrast.rs (4), grammar.rs (2). These modules are pub used from the crate root under the default ratatui feature.
• runtime: model_context/fragment.rs (5), retry_status.rs (2).
• tui: approval_log.rs, core/engine/dispatch.rs, llm_client/mod.rs (3), route_billing.rs (2), skills/mutation.rs, tui/hotbar/actions.rs, tui/transcript_cache.rs, tui/widgets/tool_card.rs (2).

hotbar/actions.rs keeps its clippy::large_enum_variant. The doc on SECONDARY_CHROME_CONTRAST said the constant only feeds the test-gate audit, "hence the dead_code allowance"; tui/whales.rs uses it at runtime, so that paragraph goes with the allow.

Left alone even though the pass found them stale:

• TurnScratch in runtime prompt_zones.rs: its comment records that rustc 1.89 and newer rustc disagree about it (cargo +1.89 check fails on origin/main: 8 unfulfilled #[expect(dead_code)] lint expectations #6543).
• 22 allows in files that other open PRs (the 0.10.2 wave among them) were changing when I ran the pass. They can go in a later slice.

Nothing here sits in a platform-gated block, and none of these items is reached only from cfg(unix) or cfg(windows) code, so the macOS result holds for the Windows build too.

Main measures 252 against a ceiling of 258. With this change it is 222, and the budget file is regenerated with --update. #6943 regenerates the same file, so whichever of the two lands second will conflict there; I'll redo --update on that branch when it happens.

Issue

Refs #5587

How I tested it

• On CI's toolchains (run with chore(tui): drop the blanket dead_code allow in five modules #6943 applied on top as well): cargo +1.99.0 clippy --workspace --all-targets --all-features with CI's flags, cargo +1.99.0 fmt --check, cargo +1.99.0 check --workspace --lib --bins, and the MSRV job's cargo +1.89.0 check --workspace. All clean.
• cargo fmt --all -- --check, and cargo check / cargo clippy (CI's flags, --all-features) for palette, runtime and tui with --all-targets: clean.
• cargo nextest run -p codewhale-palette -p codewhale-runtime -p codewhale-tui --lib: 15623 passed, 47 failed. The 47 failures are exactly the ones current main fails on my machine (most need an extension-host Node version I don't have).
• check-dead-code-budget.py passes. check-blocking-calls-budget.py fails, but it fails the same way on current main (one new std::fs site in crates/tui/src/oauth.rs), and this change doesn't touch it.

No new tests; removing an attribute that suppresses nothing does not change behaviour.

Checklist

• One focused change, rebased on current main
• cargo fmt --all passes, and tests cover new or changed behavior
• If this adds or changes a user-facing feature, I updated its docs and its row in docs/features.toml (cargo test -p codewhale-tui --test feature_registry checks it) — n/a

[claude]

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