chore(nests): mv nestsClient-browser-interop/ → nestsClient/tests/browser-interop/

Mirrors the existing nestsClient/tests/hang-interop/ layout — all
T16 cross-stack interop test infrastructure (Rust sidecars + bun
browser harness) now lives under nestsClient/tests/.

Updates:
- nestsClient/build.gradle.kts: browserInteropDir path.
- PlaywrightDriver.kt + BrowserInteropTest.kt: kdoc comments.
- All plan docs in nestsClient/plans/ that referenced the old
  path.

Compiles clean post-move. No CI changes (those jobs are
intentionally not wired per 2026-05-07-cross-stack-interop-ci-gating.md;
when re-added they'll reference the new path).

nestsClient/tests/browser-interop/tsconfig.json

@@ -0,0 +1,17 @@
+{
+  "compilerOptions": {
+    "target": "ES2022",
+    "module": "ESNext",
+    "moduleResolution": "bundler",
+    "strict": true,
+    "lib": ["ES2022", "DOM", "DOM.Iterable", "WebWorker"],
+    "types": ["@types/bun"],
+    "skipLibCheck": true,
+    "esModuleInterop": true,
+    "allowSyntheticDefaultImports": true,
+    "resolveJsonModule": true,
+    "isolatedModules": true,
+    "noEmit": true
+  },
+  "include": ["src/**/*.ts"]
+}