Summary

Headed browser test execution previously ran all test files in parallel by
firing reloads concurrently and waiting on a single shared "all tests done"
promise. This PR replaces that approach with a HeadedSerialTaskQueue that
runs one file at a time, waits for iframe frames to be ready before starting,
and applies a per-file timeout instead of a single global timeout.

Behavior diff:

+------------------+----------------------------------------------+--------------------------------------------+
|  Aspect          |  Before                                      |  After                                     |
+------------------+----------------------------------------------+--------------------------------------------+
|  Execution order |  All files reloaded concurrently             |  Files run one-at-a-time via serial queue  |
|  Readiness sync  |  No frame-ready handshake; races possible    |  Host waits for onRunnerFramesReady RPC    |
|  Timeout scope   |  One global timeout (max × file count + 30s) |  Per-file timeout (testTimeout + 30s)      |
|  iframe src      |  Always set to runner URL                    |  Set to about:blank until runId is known   |
+------------------+----------------------------------------------+--------------------------------------------+

Changes:

• headedSerialTaskQueue.ts — new module; serial promise queue that keeps
  draining on individual task failures
• hostController.ts — waitForRunnerFramesReady / markRunnerFramesReady
  handshake; per-file reloadTestFileWithTimeout; enqueueHeadedReload
  replaces direct rpcManager.reloadTestFile calls; watch-mode rerun also
  uses the queue
• browser-ui/main.tsx — notifies host via onRunnerFramesReady once frames
  are mounted; defers iframe src to about:blank until runId is assigned;
  setActive called immediately on rerun trigger
• HostRPC / HostRpcMethods — new onRunnerFramesReady method added to
  both the UI-side type and the host RPC handler