build(test): enable tsc incremental for the typecheck

tsc --noEmit supports --incremental in TS >=5.6: it emits a .tsbuildinfo
(ignored via *.tsbuildinfo) and reuses it across runs, halving the
per-check cost (0.74s -> ~0.35s) on every pnpm test since vitest runs the
typecheck first. Still catches injected src errors (verified).

Vitest's experimental.fsModuleCache was measured and rejected: cold runs
pay a ~23MB write with no gain, warm runs sit inside the run-to-run noise
band, and caching transformed modules conflicts with the suite's stated
contract that specs import src/ directly so coverage reflects what ships.

package.json

@@ -44,7 +44,7 @@
     "watch": "bash scripts/watch.sh",
     "web": "bash scripts/web.sh",
     "register": "bash scripts/register.sh",
-    "typecheck": "tsc --noEmit -p tsconfig.tests.json",
+    "typecheck": "tsc --noEmit -p tsconfig.tests.json --incremental",
     "test": "pnpm run typecheck && vitest run --coverage",
     "coverage": "vitest run --coverage",
     "release": "bash scripts/publish.sh",