Prefilter plain C headers before C++ parsing

[yzxcj797]

Summary

Header promotion introduced two throughput costs for plain C repositories:

• every .h file paid a speculative tree-sitter-cpp parse;
• every C++ file paid the per-byte Qt macro masking walk even when no Qt macros existed.

This change adds cheap source-level prefilters:

• .h files are only speculatively parsed as C++ when they contain textual evidence such as class, namespace, template, ::, constexpr, noexcept, or a trailing return arrow;
• Qt macro masking returns immediately when the source contains neither Q_ nor QT_.

Qt/C++ behavior is unchanged when those signals are present.

Fixes #802.

Testing

• Added a plain C header regression that fails if the C++ parser is probed.
• Added a C++ header regression confirming speculative promotion still happens.
• Added a direct Qt-mask fast-path regression.
• Existing Qt header behavior remains covered.
• pytest tests/test_pr802_cpp_prefilter.py tests/test_cpp_qt_headers.py -q — 17 passed
• ruff check on changed files — passes
• git diff --check — passes

[github-actions]

code-review-graph review

Overall risk: 0.40 (MEDIUM) — 8 changed function(s)/class(es), 0 affected flow(s), 2 test gap(s)

Risk-scored changes

Risk	Level	Symbol	Location	Tested
0.40	medium	code_review_graph/parser.py::CodeParser._mask_cpp_qt_macros	code_review_graph/parser.py:2839	yes
0.35	low	code_review_graph/parser.py::CodeParser._has_cpp_header_text_evidence	code_review_graph/parser.py:2828	no
0.30	low	code_review_graph/parser.py::CodeParser	code_review_graph/parser.py:2405	yes
0.30	low	code_review_graph/parser.py::CodeParser.parse_bytes	code_review_graph/parser.py:2600	yes
0.30	low	tests/test_pr802_cpp_prefilter.py::guarded_parser	tests/test_pr802_cpp_prefilter.py:13	no
0.20	low	tests/test_pr802_cpp_prefilter.py::test_qt_mask_skips_sources_without_structural_macros	tests/test_pr802_cpp_prefilter.py:33	(test)
0.05	low	tests/test_pr802_cpp_prefilter.py::test_plain_c_header_skips_speculative_cpp_parser	tests/test_pr802_cpp_prefilter.py:8	(test)
0.05	low	tests/test_pr802_cpp_prefilter.py::test_cpp_header_still_uses_speculative_parser	tests/test_pr802_cpp_prefilter.py:25	(test)

Test gaps

• code_review_graph/parser.py::CodeParser._has_cpp_header_text_evidence (code_review_graph/parser.py:2828)
• tests/test_pr802_cpp_prefilter.py::guarded_parser (tests/test_pr802_cpp_prefilter.py:13)

Token savings: this graph-backed report used ~162,901 fewer tokens (~99%) than reading every changed file in full (estimated, chars/4 approximation).

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