fix(pty): complete terminal subscription lifecycle

[jlongster]

Demonstrated subscription fixes

Stacked above #17 / windows-verification at 9f8f85c; lower branches remain frozen and unmerged.

• Cache the exact final Exited event only after real child exit and reader EOF; late observers/controllers receive it after replay/controller setup.
• Give each attachment guard a bounded disconnect signal. Dropping the guard wakes a blocked controller send even if callers retain event-receiver clones, then reliably queues Detach.
• Ignore stale queued Detach messages for a still-live replacement sharing the same string ID.
• Remove the complete failed subscriber set before promoting a replacement; disconnected guards cannot be promoted.
• Preserve ordinary controller backpressure, resize-before-input ordering, protocol7, actor-owned Ghostty, and the existing worker architecture. No unbounded detach queue, retry worker, or broader Unix I/O change.

Four small regressions cover late completion/final offset, retained receiver clones with one-slot queues, stale-ID detach, and failed-observer removal before fallback. Extensive probes/generators are not committed.

A separate test-only commit changes the existing foreground fixture to tail --pid=$$ -f /dev/null & sleep 5; every assertion/timing remains unchanged. This ties its background child to the owned shell, not a claimed repair or explanation of prior CI hangs.

Verification

• Local locked Clippy all targets/features with -D warnings and fmt check pass.
• Full Linux suite:65 passed (one ignored self-spawned fixture entry).
• Miri:5 callback +1 retained-pointer tests pass in both Stacked and Tree Borrows.
• Isolated, hash-recorded trigger probes: late observer/controller completion, saturated ordinary Drop with and without receiver clone, subsequent joined global cleanup all pass. Lifecycle/Unicode and concurrent input/resize checks also pass locally.
• Native baseline late-attach failure is independently confirmed on both Windows architectures by run33793644572.
• Native checks of THIS fix and focused remote trigger reruns are pending. Do not merge.

Scope remains the demonstrated controller-send/actor-queue cycle; this does not promise ordinary detach bypasses unrelated native/writer stalls. The excluded Linux blocked-master-write limitation is unchanged.

[jlongster]

Both subscription fixes are complete and reviewed. This head passed ordinary native x64/ARM64 (63 tests plus standalone38 each), Linux65/macOS60 and Miri6+6. The final stack in #19 also passes its ordinary native/Core suites and all eight focused regressions on x64. The extra final ARM64 focused job timed out during compilation before running probes; that explicit verification gap and the final evidence links are documented in #19. No further optional retries or merges are being performed.