fix(extraction): skip bodiless C++ forward declarations (#1093)

[colbymchenry]

Closes #1093.

Problem

In a large C++/Unreal-Engine codebase, codegraph_explore for a heavily used class (e.g. APXCharacter) returned mostly forward-declaration sites and even picked one as the blast-radius representative, while the real definition (with members and callers) was crowded out.

A class Foo; forward declaration parses as a bodiless class_specifier. extractStruct (#831) and extractEnum already skip their bodiless forms, but extractClass did not — so every forward decl (repeated across dozens of headers) minted a phantom bodiless class node competing with the single real definition. A probe confirmed it: a class forward-declared twice then defined produced 3 class nodes instead of 1.

Fix

• New opt-in skipBodilessClass?: boolean on LanguageExtractor, set only on cppExtractor.
• extractClass resolves the body once at the top and returns early when the flag is set and the node is bodiless — mirroring the existing bodiless struct/enum skip — then reuses that resolved body for the member walk.
• The flag keeps this C/C++-scoped. Languages where a bodiless class is a complete definition (Kotlin class Empty, Scala case object / trait) leave it unset and are unaffected — verified they funnel through the same extractClass path.

Validation

• Post-fix probe: C++ collapses 3 → 1 (the real definition, with its takeDamage method, survives); bodiless-struct control unchanged; Kotlin Empty/Full and Scala trait Marker/case object Red/class Foo all still indexed.
• 3 new regression tests under #1093 in extraction.test.ts (collapse to single definition, elaborated-type reference → no phantom, Kotlin/Scala unaffected).
• npm run build (tsc) clean.
• Full suite: 1863 passed, 4 skipped — the only failure was the known-flaky #662 daemon test (unrelated; passes on retry).

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