fix(parser): retain single-letter language skills

[qtjg]

PR title

fix(parser): retain single-letter language skills

PR body

Resolves #832

Summary

The skills parser previously dropped one-character programming languages because isSkillToken applied an unconditional two-character minimum. This change allowlists the defensible single-letter languages C, R, and D case-insensitively while preserving the existing noise floor for other stray one-character glyphs.

Changes

• Added a named SINGLE_LETTER_SKILLS allowlist for c, r, and d.
• Added regression coverage proving C, R, and D survive while X remains rejected.
• Removed the resolved skills knownWrong entry from the LaTeX fixture and rebaked the affected corpus snapshots.
• Updated the Google Docs fixture to retain its independent Fluent in Spanish disagreement under issue [parser] skills — a "Language: Fluent in Spanish" proficiency row is admitted as a skill token #833; the separate C-drop disagreement is now resolved by [parser] skills — single-letter language tokens (C, R, D) are dropped by isSkillToken's 2-character floor #832.
• Lowered UNFILED_TRUTH_CEILING from 9 to 8 after filing/resolving the two repeated C-drop measurements.

Verification

• Focused heuristic skills tests pass: 46 tests.
• Corpus regression tests pass: 64 tests, 1 intentionally skipped.
• npm run check:baselines completes with the expected existing unfiled-fixture warnings; the remaining filed Google Docs skill disagreement is linked to [parser] skills — a "Language: Fluent in Spanish" proficiency row is admitted as a skill token #833.
• Typecheck, lint, and production build pass.
• The full npm run verify run reached 358 passing test files and 5,864 passing tests, but is blocked by an unrelated existing useLibraryChanges BroadcastChannel/jsdom failure with three unhandled MessageEvent errors. No issue [parser] skills — single-letter language tokens (C, R, D) are dropped by isSkillToken's 2-character floor #832 tests fail.