fix(filter/tsc): severity-rank clusters + per-severity header

[Asgarrrr]

Summary

V0.4.1 polish round, item #1 of 4 (severity rank → quarantine → overload truncation → cache-stable header). Lands the deferred § 9c-b item from docs/plans/filter-engine/00-decisions.md before V0.5 filters arrive so the engine pattern is consistent from day 1 of the next roster.

Bug fixed

Errors get buried under larger warning clusters today. With 1 TS2322 error and 30 TS6133 unused-var warnings, the bullet order is:

- **TS6133** ×30 — same shape: ...    ← warnings first because cluster is largest
- **TS2322** src/auth/login.ts:42:3 — Type 'string' is not assignable to type 'number'.

Now:

- **TS2322** src/auth/login.ts:42:3 — Type 'string' is not assignable to type 'number'.    ← error surfaces first
- **TS6133** ×30 — same shape: ...

Header rewrite

Today: # tsc — 33 errors in 4 files (Tms) — conflates 3 errors and 30 warnings under "errors".

After: # tsc — 3 errors, 30 warnings in 4 files (Tms). Singularizes per-severity (1 error, 2 warnings); falls back to bare N errors when only errors are present. info stays singular regardless of count.

Implementation

• cluster.ts — primary sort key on severity (error → warning → info), tiebreakers unchanged (members.length desc, ruleId, first-member file).
• render.ts — count by severity instead of total; build header from non-empty severity parts.
• index.ts — bump TSC_FILTER_VERSION tsc/2 → tsc/3.

Test plan

• bun test — 317/317 pass (+5 new: 2 cluster severity-rank tests, 3 render header tests)
• bun x biome ci . — clean
• bun run typecheck — clean
• Existing fixtures (small/medium/large.txt) are pure errors → snapshots unchanged

Risk / rollback

Output shape changes only when a run has multiple severities. None of the committed fixtures do, so snapshots are byte-identical and the bench compression numbers stay valid. The tsc/3 bump means archived tsc/2 evidence carries the pre-rank ordering attribution. Rollback: revert this commit, no migrations.