Browser router phase 1 follow-ups from PR #121 review #122

PR #121 (merged as 8037810) shipped the browser tool router and the external Playwright tier. This is phase 1, PR 1 of the browser tiers design in docs/adr/browser-tiers.md and docs/requirements/browser-tiers.md (issue #118). Review found no HIGH findings. This issue collects the MEDIUM and LOW follow-ups.

Out of scope here and tracked in #118: the stdio managed Playwright child (later phase 1 PRs), the wizard page, on-demand install, and the Apple Events tier.

MEDIUM

• 1. PortOwnerProbe matches only the local port. It checks neither address family nor loopback address, so a same-uid listener on 127.0.0.1:<port> can approve a request to ::1:<port>. Pass the validated endpoint into the probe. Add a test with simultaneous IPv4 and IPv6 listeners.
• 2. ExternalPlaywrightClient.healthCheck re-runs initialize every 30 s.
  • It overwrites the session id.
  • It never sends DELETE for old sessions.
  • It uses the 90 s request timeout, while R2 allows 2 s.

LOW

• Health check timeout should be 2 s (R2), not 90 s (PlaywrightBackend / ExternalPlaywrightClient).
• BrowserRouter does three owner probes plus an extra tools/list per tools/call. Reduce.
• PlaywrightBackend.tools() turns a transient error into []. Propagate or keep the last known list.
• R1.6 and R7.5.5 reporting is not wired up: mode and ownerSummary are missing from sys_info and the menu, and there is no structuredContent.backend.
• The BrowserRouter guard map last is never pruned.
• The browser upstream port is not range-checked in main.swift.
• A repeated --upstream browser= flag is dropped silently. Reject it or take the last value, and say so.
• The Host header for ::1 is sent without brackets. It should be [::1]:<port>.
• The BrowserCatalogTests tailscale scan is case-sensitive, while the shell guard uses grep -i. Make the Swift scan case-insensitive.