Fix cpp class_start forward-declaration false positive (closes #2011)

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Summary

Fixes #2011, found during the cpp tri-comparison-ledger-sweep merged in #2015. That
PR fixed the comparison tooling's own tree-sitter walker (which had the identical
lang == "c"-only forward-declaration guard gap as GitGalaxy's own detector.py),
but deliberately left the production detector.py fix filed rather than patched
inline, since a naive copy of C's existing flat lookahead regex is unsafe for cpp.

That decision turned out to have a real, immediate consequence: once the tree-sitter
walker stopped agreeing with GitGalaxy's forward-declaration false positives, main
was left with a standing inconsistency — tests/tree_sitter_accuracy_baseline_cpp.json
still commits the old, both-sides-wrong numbers (real_classes: 171, found_classes: 149), while the actual tree-sitter ground truth now computes 65. Any PR touching cpp
tooling would inherit this as a tree-sitter-accuracy-audit CI failure through no
fault of its own (confirmed: this is what happened while finishing out PR #2015's own
review). This PR closes that gap by actually fixing the underlying bug rather than
leaving main in a broken state.

The bug: GitGalaxy's own class_start regex counted a bare forward declaration
(class AudioStreamPreviewGenerator;, godot/editor_node.h:68 and 83 more of the same
shape in the same file) as if it were a real class definition —
_CLASS_START_REQUIRES_BODY_ANCHOR in detector.py, the exact mechanism that already
guards against this for C, was never extended to cpp.

Why not just copy C's fix: C's existing lookahead (re.search(r"^[^\{;,)=]{0,200}?([\{;,)=])", ...))
stops at the first {/;/,/)/=. C++ multiple inheritance
(class Foo : public A, public B {) hits the comma stop-char before the real {,
falsely excluding a legitimate multi-inheritance class definition — confirmed via
direct regex testing before this fix was written specifically to avoid that
regression.

The fix: _cpp_class_has_body(), a depth-aware scanner (paren/bracket/angle-bracket
tracking, same style as this file's existing _dart_scan_terminator/
_count_top_level_args helpers) that correctly walks through an inheritance clause's
own top-level commas and template args before checking for a real body opener. A
top-level , before any inheritance-list : has been seen is still treated as a
non-definition signal, preserving the pre-existing protection against a type-use in a
declarator list (struct Foo *a, *b;) or a function parameter default
(void f(struct Foo* p = nullptr)).

Also regenerates tests/tree_sitter_accuracy_baseline_cpp.json (class recall 87.1% →
100.0%, both found_classes and extra_classes now correctly reflect zero forward-
declaration false positives) and the tri-comparison ledger/chart/docs/language_status/ cpp.md to record the fix.

Test plan

• 11 hand-built regression cases for _cpp_class_has_body (forward decl, plain
  definition, single/multi inheritance, templated bases, type-use-in-declarator-
  list, function-parameter-default, final specifier) — all pass
• Full cpp extraction gauntlet (122 tests) + tests/core_engine/test_detector.py
  (141 tests) + C's own extraction gauntlet (128 tests, confirmed unaffected)
• python tests/ruff_audit.py --ci / mypy_audit.py --ci — no new findings
• ruff format --check clean
• tests/tools/crucible_check.py (full-precision + zero-dependency) against the
  full ~80-repo corpus — zero golden-master diff, confirmed legitimate (not a
  blind spot) via a direct DB query showing the named-class list this fix touches
  dropped from ~95 to 11 for editor_node.h specifically
• tree_sitter_accuracy_audit.py --lang cpp --ci — passes cleanly against the
  regenerated baseline

🤖 Generated with Claude Code

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