audit: flex review and fixes

[theodevelop]

Type of change

• Bug fix

What does this PR do?

Proactive Flex audit — 4 fixes found and corrected on branch audit/flex-review:

• Bug A (diagnostics.ts — rawPattern()): spaces inside Flex quoted strings "hello world" were treated as pattern/action delimiters, causing false flex/unreachable-rule for distinct patterns sharing a common word prefix. Fixed by tracking inQuote depth.
• Bug B (flexParser.ts): a standalone { on its own line (multi-line action syntax) was pushed as a spurious rule entry, causing false flex/unreachable-rule for the second and subsequent multi-line-action rules. Fixed by detecting trimmed === '{' and setting actionDepth = 1.
• Bug C (flexParser.ts): all SC-related regex patterns used [A-Z_][A-Z0-9_]* (uppercase only). Lowercase SC names like comment were silently ignored, skipping all flex/undefined-sc / flex/unused-sc diagnostics for them.
• Bug D (flexParser.ts): the single-tab pattern-action separator (pattern\taction) was not recognised by the old \s{2,} heuristic, so {identifier} tokens in a C action body were falsely counted as abbreviation refs, suppressing flex/unused-abbrev.
• Cleanup (diagnostics.ts): removed two dead entries in CATCHALL_PATTERNS that contained a literal newline character and could never match a rule line

Related issue

Closes # (audit, no issue number)

How to test manually

Open tests/_test.l in VS Code — no false diagnostics should appear. Verify that GREET1/GREET2 (defined but not used as {abbr}) correctly show flex/unused-abbrev.

Checklist

• npm run compile passes with no new errors
• Tests added or updated (npx ts-node ...)
• Manual test done in VS Code
• CHANGELOG.md updated
• No unintended files staged (node_modules, .env, dist...)