build(js): upgrade TypeScript to 6.0 (#457)

Bump typescript to ^6.0.3 in clients/js. TS6 requires an explicit
rootDir for declaration emit, so set "rootDir": "./src" in
tsconfig.declarations.json (without it, `tsc -p tsconfig.declarations.json`
fails with TS5011). The client already uses the modern "bundler" module
resolution, so no resolver change or deprecation suppression is needed.

Verified locally: `pnpm build` (tsup + tsc declarations) and `pnpm lint`
pass under typescript@6.0.3.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: clankmaxxing-clod

clients/js/package.json

@@ -55,7 +55,7 @@
         "rimraf": "^6.0.1",
         "tsup": "^8.1.2",
         "typedoc": "^0.28.14",
-        "typescript": "^5.9.3",
+        "typescript": "^6.0.3",
         "vitest": "^4.0.15"
     },
     "peerDependencies": {