P1: Isolate terminal replies, drafts and command history when switching threads #1182

Audit #1180, 2026-09-10. Baseline: checkout 9fc8645; fetched origin/main bf6fb02 has identical relevant implementation. This is implementation backlog, not a shipped fix.

Reproduced

Mounted the actual TerminalPane with the existing DOM harness:

1. Open A, submit echo A, and defer the write promise.
2. Switch the same pane to B; its open reply displays B.
3. Resolve A's write with non-reset text "late A" and its new cursor.
4. B now contains A's output. ArrowUp in B also recalls echo A.

Both outcomes were asserted against the rendered component. This is a deterministic simulated-transport check, not a desktop screenshot test.

Cause

src/components/TerminalPane.tsx: submit/restart promise chains call applyState without checking the current thread/session generation. Open/poll have cleanup guards; mutations do not. The thread-change effect also retains draft and history refs. Late replies can overwrite B's cursor, cwd and running state. Rejected writes clear the draft and silently swallow errors.

Acceptance

• Old thread/restart replies never alter the active pane, including A to B to A.
• Scope history and preserve unsent command drafts by thread.
• Recover the draft on rejected write and show actionable errors for write/restart failures.
• Cover delayed write/restart, thread switches and failed submission.
• Preserve cursor/reset recovery and integrate the same-session overlap work from Overlapping terminal reads duplicate scrollback on slow connections #934.

Distinct from #934: that ticket concerns overlapping deltas within one session. This concerns session ownership and unsent command state. Check its landed branch before editing shared cursor handling.