fix(dead-code): stop false-flagging Rust modules/impl-blocks/trait-defaults as dead (closes #228)

[Wolfvin]

Root cause

_detect_dead_from_registry in scripts/deadcode_engine.py flags every backend node with ref_count == 0 && status == "dead". Three Rust structural/implicit node kinds always have ref_count == 0 in the raw call graph because they never appear as a CALLS target, yet they are never dead:

1. Module declarations (mod foo { ... }, type == "module") — a namespace referenced by path, not called. Previously only mod tests was exempted.
2. Impl blocks — synthetic node impl X (type == "impl", fn == "impl_X", id file.rs:LINE:impl:X). The block is a container; only its methods should be analyzed individually.
3. Trait-default methods (default, clone, fmt, drop, eq, hash, …) — invoked implicitly via derive macros, generic trait dispatch, and ..Default::default() spread, which the CALLS extractor cannot observe.

(The #[cfg(test)] mod tests inline-test case was already fixed earlier; this closes the residual gap.)

Fix (only scripts/deadcode_engine.py)

• Skip any node with type in ("module", "impl") inside the dead branch — structural containers, never callable.
• Add a targeted _RUST_TRAIT_DEFAULT_METHODS whitelist; skip Rust fns whose bare name is on it. Narrow by design — any standalone fn not on the list stays eligible.

Verification

New regression test tests/test_issue228_rust_impl_dead.py (mirrors the _run_scan harness of test_graph_accuracy_golden.py) with a fixture containing a mod, an impl with a used method, an impl Default { fn default() }, and a genuinely-dead standalone fn orphan_helper.

$ python -m pytest ../tests/test_issue228_rust_impl_dead.py -q
....                                                                     [100%]
4 passed

The control assertion test_genuinely_dead_fn_is_flagged proves orphan_helper IS still reported dead — the filter narrows false positives without disabling genuine detection.

Full dead-code suite (-k "dead or deadcode or issue228"): 56 passed, 1 failed. The single failure (test_confidence.py::...test_dead_code_json_output_has_schema_version) is pre-existing — it fails identically on the base commit (verified via git stash) and is unrelated to this change.

🤖 Generated with Claude Code

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[sonarqubecloud]

Quality Gate passed

Issues
2 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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