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/plans/2026-05-07-cross-stack-interop-ci-gating.md

@@ -73,13 +73,13 @@ browser-interop:
     - uses: actions/cache@v4
       with:
         path: |
-          nestsClient-browser-interop/node_modules
-          nestsClient-browser-interop/dist
-        key: ${{ runner.os }}-bun-${{ hashFiles('nestsClient-browser-interop/package.json', 'nestsClient-browser-interop/bun.lock') }}
+          nestsClient/tests/browser-interop/node_modules
+          nestsClient/tests/browser-interop/dist
+        key: ${{ runner.os }}-bun-${{ hashFiles('nestsClient/tests/browser-interop/package.json', 'nestsClient/tests/browser-interop/bun.lock') }}
     - uses: actions/cache@v4
       with:
         path: ~/.cache/ms-playwright
-        key: ${{ runner.os }}-playwright-${{ hashFiles('nestsClient-browser-interop/package.json') }}
+        key: ${{ runner.os }}-playwright-${{ hashFiles('nestsClient/tests/browser-interop/package.json') }}
     - name: Run browser cross-stack interop suite
       run: |
         ./gradlew :nestsClient:jvmTest \
@@ -92,8 +92,8 @@ browser-interop:
         name: Browser Interop Test Reports
         path: |
           nestsClient/build/reports/tests/jvmTest/
-          nestsClient-browser-interop/test-results/
-          nestsClient-browser-interop/playwright-report/
+          nestsClient/tests/browser-interop/test-results/
+          nestsClient/tests/browser-interop/playwright-report/
 ```
 
 Same `git show b94737de7 -- .github/workflows/build.yml` reverse