fix(pty): serialize xterm screen writes

[stevenahhh]

Summary

• serialize TerminalScreen writes through xterm parse callbacks
• order resize recreation and replay with concurrent feeds
• keep the queue usable after an individual write rejection

Root cause

TerminalRuntimeSession intentionally does not await screen.feed(), so fast PTY output could enqueue more than xterm's 50 MB pending-write watermark. xterm then rejected a feed with write data discarded, use flow control to avoid losing data; the discarded promise became an uncaught exception and exited Senpi.

Verification

• regression test fails before the fix with the exact xterm error
• npm test -- test/screen.test.ts: 45 passed, 7 skipped
• npm run check: passed
• 20,000 concurrent 4 KB feeds: 20,000 fulfilled, 0 rejected
• .agents/skills/senpi-qa/scripts/pty-drive.mjs --self-test: 8/8 passed (pipe fallback on Windows)

Related xterm guidance: https://github.com/xtermjs/xtermjs.org/blob/f8670249e22dac222c428654d9fb39a957555f0b/_docs/guides/flowcontrol.md

[chatgpt-codex-connector]

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