fix: bound C++ extractor regex backtracking

[AustenMan]

Problem

C++ signature extraction can spend hours in catastrophic regex backtracking on qualified-name declarations. In a 501-file C++ scan, generated/include/viewer/automation/api_types.hpp was the first reproducible stall; a 21-segment synthetic declaration took 3.854 seconds by itself.

Fix

Make the repeated C++ return-type prefix atomic so failed function-declaration matches cannot repartition the same qualified tokens exponentially. Add an adversarial regression test with a generous timing bound.

Evidence

• Synthetic adversarial declaration: 3.854 s to 0.000019 s
• Previously stalling api_types.hpp: 0.055901 s
• All 501 C++ files: 5.261096 s, 2,263 functions
• C++ plugin and generic signature detector tests: 64 passed
• Focused extractor tests: 6 passed