Detect export-macro class/struct .h headers as C++, not C (#1159)

[nftmago]

Fixes #1159.

Problem

A .h whose only C++ tell is an all-caps export/visibility macro between the keyword and the type name — the ubiquitous Unreal Engine / library form class ENGINE_API Foo : public Bar — was classified as C by looksLikeCpp. Parsed with the C grammar, the whole class definition (node, members, base clause) never materialized in the graph, breaking type-hierarchy, callers, and blast-radius queries through that type.

This sits one step upstream of the existing #1061 export-macro blanking: that recovery never runs, because detection has already routed the header to C.

Note the issue's minimal repro also has public:, which looksLikeCpp already catches via another branch — so the truly-failing case is a macro class carrying no other C++ tell in the first 8 KB. Confirmed empirically before/after:

class ENGINE_API Foo : public Bar { void Tick(); };   // before: c   after: cpp
struct CORE_API Widget : Base { int n; };             // before: c   after: cpp
struct Point { int x; int y; };                        // before: c   after: c  (guard)

Fix

looksLikeCpp's class/struct signal now tolerates an optional all-caps macro between the keyword and the name. struct is matched only in its base-clause form (struct X : Base) — that : is C++-exclusive, whereas a plain struct X { … } is valid C and must stay c.

Tests

Added to extraction.test.ts: detection for macro class (with/without base) and macro struct; regression guards (plain class:base stays cpp, plain C struct{} stays c); and an end-to-end case proving a macro-only header now recovers the class node and its extends edge (detection + #1061 blanking together).

Full suite green (124 files, 2121 passed).