parser: tag CALLS edges inside dead guards with reachable flag

[HouMinXi]

Linked issue

Closes #576

What & why

Calls inside if False: and if TYPE_CHECKING: blocks are statically
unreachable, but the parser emits CALLS edges for them anyway. This
causes false positives in dead-code detection (refactor_tool mode
dead_code) and inflates impact-radius results.

This PR adds a lightweight ancestor-walk helper
_is_in_static_dead_guard() that detects these two common Python
idioms and sets extra["reachable"] = False on the affected CALLS
edges. Live edges omit the key entirely, so:

• Existing graph.db files remain forward-compatible (absent key =
  live).
• Downstream consumers can filter with:
  json_extract(extra, '$.reachable') IS NULL OR json_extract(extra, '$.reachable') != 0

Scope

• Covered (v1): All Tree-sitter languages routed through
  _extract_calls (Python, JS/TS, Go, Rust, C, C++, Java, etc.).
• Not covered: Bespoke handlers (ReScript, Dart regex path,
  Elixir), C preprocessor guards (#if 0). These are left for
  follow-up.

Design choice: ancestor walk vs threaded flag

Chose ancestor walk (approach b from #576 discussion) over threading
a flag through _extract_from_tree because it requires zero signature
changes, keeps the diff minimal, and is O(scope-depth) per call node
-- negligible overhead.

How it was tested

uv run pytest tests/test_parser.py -v          # 104 passed
uv run pytest tests/ --tb=short -q             # 1385 passed, 0 failed
uv run ruff check code_review_graph/parser.py  # All checks passed

Bug-inject verification: neutralized the guard check (if False and ...) -- test failed on dead_false_call edge missing the flag.
Restored -- test passed.

Real-path smoke test: parsed a Python file with if False: and
if TYPE_CHECKING: calls outside the test suite; confirmed live calls
show reachable=ABSENT and dead calls show reachable=False.

Checklist

• Tests added for new functionality
• All tests pass: uv run pytest tests/ --tb=short -q
• Linting passes: uv run ruff check code_review_graph/
• Lines are at most 100 characters
• Type checking passes (mypy not run locally; CI will verify)
• Docs updated where behavior changed (no user-facing API change)

[itxaiohanglover]

Nice implementation! I reported #576 and this is exactly what I had in mind. The scope boundary check (stopping at function/class/module) is the right call — a dead guard in an outer scope shouldn't propagate to nested definitions. Tagging with reachable: False rather than removing edges is smart too, keeps the graph complete for downstream consumers.

One small thing: might be worth handling if 0: as well? It's less common but I've seen it in C-extension bindings. Not a blocker though.

[HouMinXi]

Good catch -- added if 0: support in ded4913. Tree-sitter parses 0 as an integer node with text == b"0", so the check mirrors the existing false node pattern. Test fixture and assertions updated accordingly.