fix: extended fence parsing and --update block alignment

[jlevy]

Summary

• Parser now supports 4+ backtick fences (CommonMark extended fences), allowing nested triple-backtick code blocks in expected output.
• Fix --update rewriting wrong block when --filter/<!-- only --> runs a subset of tests (same index-mapping bug previously fixed in --expand).
• Updater and expander preserve original fence length when rewriting blocks.

Test plan

• 3 new parser tests: extended fence parsing, nested triple backticks, fence-length boundary
• 2 new updater tests: subset alignment, extended fence preservation
• All 118 unit tests pass
• pnpm precommit (format, lint, typecheck, test) passes
• pnpm build && pnpm publint passes

Closes try-319, try-dz8r.

Made with Cursor

[github-actions]

Coverage Report

Status	Category	Percentage	Covered / Total
🔵	Lines	94.52%	3276 / 3466
🔵	Statements	94.52%	3276 / 3466
🔵	Functions	44.44%	80 / 180
🔵	Branches	42.68%	370 / 867

File Coverage

No changed files found.

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