Remove stale Rust scaffolds and blanket dead-code allowances

[arniber21]

What to build

Delete or finish stale implementation artifacts that no longer contribute to a compiled interface, and replace crate-wide dead-code suppression with narrow, justified allowances.

Acceptance criteria

• The orphaned duplicate compaction types are removed or made the single compiled source of truth.
• No-op ZX rewrite placeholders are implemented, deleted, or represented as explicit unsupported capabilities outside production code.
• Stale “stub implementation” crate documentation reflects the current implementation.
• Crate-wide allow(dead_code) directives are removed.
• Remaining dead-code allowances are item-local and explain the active caller or external contract.

Blocked by

None - can start immediately.

[arniber21]

Implemented and merged via PR #455. Closing as complete.