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/build.gradle.kts

@@ -232,7 +232,7 @@ tasks.withType<Test>().configureEach {
 // ---- Cross-stack interop: BROWSER (Phase 4 of T16) --------------------------
 //
 // Adds the bun + Playwright + headless Chromium harness at
-// `nestsClient-browser-interop/`. Mirrors the hang-interop wiring above
+// `nestsClient/tests/browser-interop/`. Mirrors the hang-interop wiring above
 // but with bun/npx subprocesses instead of cargo. Opt-in via
 // `-DnestsBrowserInterop=true`. See:
 //   nestsClient/plans/2026-05-06-phase4-browser-harness.md
@@ -249,7 +249,7 @@ tasks.withType<Test>().configureEach {
 // paths the agents/host runner ship with.
 
 val browserInteropDir =
-    rootProject.layout.projectDirectory.dir("nestsClient-browser-interop")
+    rootProject.layout.projectDirectory.dir("nestsClient/tests/browser-interop")
 
 // `bun` lives at `/root/.bun/bin/bun` on the agent runner. CI may put it
 // elsewhere; allow override via env / system property. Falls back to