chore: triage dead_code-family warnings (#854)

[proggeramlug]

Summary

Closes #854. Triages the ~99 dead_code-family warnings (fields/variants/functions/methods/constants/statics never used, plus a few unused locals) that were carved out of the warning-cleanup effort.

Per the issue's guidance, each site got case-by-case judgment with a conservative bias: symbols that are part of an externally-meaningful contract or an in-progress subsystem are kept with #[allow(dead_code)] + a // #854: one-line justification rather than deleted. Only genuine, self-contained cruft was removed.

What was kept (with #[allow(dead_code)] + reason)

• GC fallback / verification paths — sweep, trace_array/object/closure, gc_type_*, validate_gc_type_*. These back the PERRY_GEN_GC=0 full mark-sweep bisection path and are exercised by gc/tests.
• NaN-boxing / value-encoding helpers — nanbox_pointer_f64, box_ptr (documented value-encoding contract, see CLAUDE.md).
• In-progress subsystems — native-ABI, typed-feedback, buffer-view, pod-record taxonomy variants/helpers whose emit sites aren't wired yet.
• Deserialized wire shapes — publish/server_api, perry.toml config structs, OAuth responses. Not every field is read on every path; used field-level allows so no derive(Deserialize/Debug/Clone) is broken.
• cfg-gated / FFI surface live only on other targets (e.g. read_registry_kits_root_10, android-only path_str_for_log).

What was deleted (self-contained, zero refs)

• A shadowed let blk local in perry-codegen/src/expr/calls.rs (immediately re-bound in the inner block).
• An unused #[cfg(feature = "websocket")] js_ws_process_pending extern decl in async_bridge.rs (the drain path has its own decl).

Unused locals that are pure derived values were _-prefixed (_is_library_output, _promise_resolve, _call, _callee_expr, _keys_array, _path_str_for_log).

Validation

• cargo build --release → 0 dead_code-family warnings (down from ~99), exit 0.
• rustfmt --check passes on all 54 changed files.
• No file-level deletions; net diff is +274 / −12 across 54 files (almost entirely attribute + comment additions).

Notes for the maintainer

• Per the worktree-PR convention, this branch does not bump Cargo.toml/CLAUDE.md version or touch CHANGELOG.md — fold those in at merge.
• A handful of fields look like genuinely redundant duplicated state (flagged in code review, not acted on here): RedisClient.url (mirrors a URLS side-map), LoweringContext.next_global_id/globals (globals tracked elsewhere), ClassSpecRequest.new_name (driver re-derives the name). Left as #[allow] for a future cleanup rather than risk behavior changes.