Fix catastrophic backtracking in C++ review regex

[AustenMan]

Summary

• make the repeated C++ return-type prefix atomic so qualified-name non-matches cannot trigger catastrophic regex backtracking
• add a regression test for the pathological qualified-name assignment shape

Reproduction

For a synthetic non-match shaped as repeated qualified names followed by an assignment, the prior regex grew from 0.04 s at 16 segments to 2.9 s at 22 segments. Five real C++ files in a 447-file project failed to finish within a two-second per-file probe.

Validation

• python -m pytest desloppify/languages/cxx/tests/test_review.py -q: 3 passed
• Ruff check and format check pass for both changed files
• patched parser processed all 447 project C++ files in 1.203 s, with no file above 0.05 s
• the previously stalled project review --prepare --dimensions design_coherence completed and generated its packet

The broader desloppify/tests run completed locally with 52 failures outside the modified C++ language module; the focused modified-module suite is clean.