Fix fixed Browser debugger port publication race

[YueZh127]

Chromium can create DevToolsActivePort before its contents are written. The fixed Browser worker previously parsed the file as soon as it existed, intermittently returning error before navigation. Wait for a newline-terminated port record within the existing execution deadline, and reject complete malformed or out-of-range ports.

Fixes #855.

The readiness wait stops when Chrome exits or the original deadline expires. No timeout increase, navigation retry, new Browser capability, authorization change, or relaxed cleanup check is introduced. The runtime/result schemas and independent Host grant remain unchanged.

Validation:

• Three deterministic readiness tests cover missing/empty/partial publication, complete invalid records, non-ENOENT IO failure, deadline expiry and exited Chrome.
• Full fixed Browser suite with real Chrome: 38 passed, 0 skipped, including policy rejection, title mismatch, timeout, exact replay, concurrent delivery, killed-Host recovery and ownership failure.
• git diff --check passed. Only the fixed Browser worker and its existing test file change.
• Downstream reproduction and follow-up: YueZh127/product-quality-loop#98, PR Bind Project Profiles to explicit execution approval #97. Production acceptance remains on an exact verified pin; no diagnostic checkout is used as pinned evidence.