perf: bound mobile terminal WebView queues

[nwparker]

Summary

• bound pre-ready mobile terminal WebView write backlog by bytes and message count while preserving queued control messages
• drain embedded xterm writes with an indexed queue instead of Array.shift()
• dispose old xterm observer subscriptions on terminal replacement and clear successful ready/measure fallback timers

Validation

• pnpm test (mobile)
• pnpm exec tsc --noEmit (mobile)
• pnpm lint (mobile)
• git diff --check

Risk: low. The patch keeps the existing write order for retained chunks and intentionally leaves the after-drain readiness barrier behavior unchanged.