Fix remote tmux seed/live output ordering

[austinywang]

Summary

• make each remote-tmux capture/state cycle a UUID-correlated pane-seed transaction
• reconcile live %output around the authoritative capture boundary instead of depending on SSH latency or read chunks
• filter authoritative snapshots independently from the stateful live title stream, then replay post-capture output exactly once
• bound pending seed memory and use the existing reconnect/reseed path on overflow
• keep lifecycle failure/rollback on one shared pending-command transaction path

Root cause

The mirror delivered live output while capture-pane and pane-state replies independently repainted the same surface. Which write won depended on transport timing, and a live screen-title filter could be left mid-escape when synthetic snapshot bytes arrived. Tailscale SSH exposed both ordering assumptions through different latency and chunk boundaries.

Regression coverage

The test-only first commit feeds the real incremental control parser with deterministic uneven transport chunks and covers:

• typed echo split around the capture result
• an ESC k screen-title sequence split across the snapshot boundary

The fix commit adds typed-seed observer coverage, including the no-duplicate compatibility contract.

A manual CI run was started on the test-only commit to record red/green status, but the existing strict determinism guard failed first on three unrelated sleep-then-assert findings in MobileShellRenderGridLivenessTests.swift. That preflight skipped the app-host tests, so the PR keeps the requested two-commit test/fix structure without claiming that unrelated CI red as regression proof.

Validation

• tagged Debug build: ./scripts/reload.sh --tag issue-7990-tailscale-mirror-echo
• Swift warning budget: 0 new cmux-owned warnings
• pbxproj normalization/object-version guard
• cmuxTests project wiring lint: 529 files checked
• swiftc parse and git diff check
• line budget: both existing 500+ line files shrink (990 to 984; 609 to 590); every new Swift file is under 500 lines
• localization audit: no user-facing strings introduced

Closes #7990

Summary by CodeRabbit

• New Features

  • Added an optional pane-seed observer callback to deliver authoritative pane snapshots with ordered live cutover, including seeded output routing.
  • Enhanced pane capture/reseed to use correlated pane-seed IDs and support optional scrollback clearing.

• Bug Fixes

  • Improved pane-seed buffering/cutover to prevent early/duplicate live output and ensure exact-once, in-order replay (including reconnect and title-escape boundary safety).
  • Refined window/surface repaint behavior for verified single-pane grid growth.

• Tests

  • Added regression coverage for uneven transport chunking, blank lines inside command blocks, reconnect/cutover edge cases, and pane grid growth command sequencing.