chore: triage dead-code warnings (fields, variants, constants, dead assignments)

[TheHypnoo]

Roughly 60 dead_code-family warnings remain after the warning-cleanup PR. Each one needs case-by-case judgment between #[allow(dead_code)] (with a one-line comment explaining why) and deletion.

Categories

• Fields never read — e.g. url, stage, id, algo, description, cancelled, expires_in_secs, estimated_wait_secs, inferred, is_dylib, job_id, module_name, name, new_name, watchos, build.
• Variants never constructed — DynamicPropertyAccess, UnsupportedSyntax, Low.
• Constants never used — POINTER_TAG, STRING_TAG, POINTER_MASK, SHORT_STRING_TAG, TAG_MARKER, TRANSITION_CACHE_MASK.
• Statics/functions never used — CALLBACK_COUNTER, with_base_dir.
• Dead assignments — value assigned to X is never read and variable X is assigned to, but never used: dep_issues_count, extract_ok, host, hostname, modifiers, n, pathname, removed_count, result_len, user_func_idx.
• Unused variables — ty, promise_resolve.

Why per-site review

Some of these are intentional:

• NaN-boxing tag constants (POINTER_TAG, STRING_TAG, POINTER_MASK, SHORT_STRING_TAG, TAG_MARKER) are part of the value-encoding contract documented in CLAUDE.md. They may be referenced from documentation, external tooling, or codegen even if rustc doesn't see a direct use.
• Struct fields can be intentional for binary layout, FFI ABI, or future-feature placeholders.
• Dead assignments may be a missing read (real bug) or a refactor leftover (safe to remove).

Others are genuine cruft that should just be deleted.

How to approach

For each warning:

• If the symbol is part of an externally-meaningful contract → #[allow(dead_code)] with a one-line comment explaining why (e.g. // part of the NaN-boxing tag contract, see CLAUDE.md).
• If the symbol is leftover from a removed feature → delete it.
• If the warning is a dead assignment, check whether the missing read is a bug or whether the assignment itself is dead.

Site list

Captured from:

cargo build --release 2>&1 | grep -B0 -A1 "is never\|never constructed\|assigned to, but never used\|value assigned to.*never read"

Context

Carved out of the warning-cleanup PR (chore: reduce compile warnings 325 → 125).