Commit 6: Add POSIX-bracket extension tests (pre-fix, capturing incorrect tokenization)

Add new test cases to test.cpp exercising POSIX bracket expressions inside
character classes as supported by std::regex ECMAScript mode:
- Single POSIX classes: [[:alpha:]], [[:digit:]], [[:space:]], [[:upper:]],
  [[:lower:]], [[:alnum:]], [[:print:]], [[:punct:]]
- Negated outer bracket: [^[:space:]]
- Mixed: regular char + POSIX class [a[:space:]]
- POSIX collating symbol [[.a.]]
- POSIX equivalence class [[=a=]]
- Mixed POSIX + range [[:alpha:]0-9]

The existing r_posix1-r_posix4 cases remain untouched. New cases use distinct
names (r_posix_alpha, r_posix_digit, etc.) to avoid redeclaration.

Generated .expected via codeql test run --learn (CodeQL 2.26.1); all 2 tests
pass. The expected output captures the CURRENT (pre-fix) tokenization — commit 7
will change these rows when correct POSIX nesting is implemented.

Author: Copilot