chore(tsconfig): set explicit rootDir to ./src for TypeScript 6.0 compatibility (#48)

TypeScript 6.0 made `rootDir` a required option when the source layout
has files outside `outDir`'s implied root (which is our case — we have
tests/ alongside src/, and `outDir` is ./dist). On TS 5.x the inference
worked silently; on TS 6.0 it errors:

  tsconfig.json(13,5): error TS5011: The common source directory of
  'tsconfig.json' is './src'. The 'rootDir' setting must be explicitly
  set to this or another path to adjust your output's file layout.

Adding `"rootDir": "./src"` is forward- and backward-compatible: it
matches what TS 5.x was inferring, so build, type-check, lint, and test
behavior on the current pinned TypeScript (5.9.x) is unchanged. Verified
locally: \`npm run build\`, \`npm run type-check\`, \`npm run lint\`, and
\`npm test\` (5490 tests) all green at this commit.

Unblocks #34 (typescript 5.9.3 → 6.0.2 dependabot bump): once this PR
lands and dependabot rebases #34, its build step will succeed.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: sjsyrek

tsconfig.json

@@ -11,6 +11,7 @@
     "declarationMap": true,
     "sourceMap": true,
     "outDir": "./dist",
+    "rootDir": "./src",
     "removeComments": true,
     "newLine": "lf",