fix(pty): make terminal cleanup complete under backpressure

[jlongster]

P2 — termination and cleanup

Complete at c92abf2, stacked on P1 #12 / conpty-runtime64427e2, above corrected T2 windows-pipes3ebd4a5. Source is frozen for T3 convergence. Do not merge.

Runtime changes

• The actor hands the unique Windows PTY master to the existing wait worker for blocking ClosePseudoConsole; actor/reader keep draining until real EOF before publishing Exited. No fifth worker or fake EOF.
• A per-terminal disconnect signal interrupts demonstrated full controller/input queue shutdown deadlocks, prioritizing shutdown without changing ordinary backpressure or resize-reply-before-input ordering.
• Late writer failures cannot overwrite a published exit. Windows post-exit input/resize returns child-exited error; snapshots/rows/replay remain readable.
• Shutdown joins are idempotent. Windows explicitly waits for the owned root process object to signal before accepting the exit status, fixing a reproduced teardown race and preserving exit code259.
• Failed close handoff retains master ownership; a regression covers it.

Exact-head verification

Windows run33786722949: native x64 AND ARM64 each passed29 library +5 callback +12 real runtime +1 retained-pointer tests (47 total), plus standalone library29/29. Architecture/PE checks, formatting, locked all-target/all-feature Clippy, build and executable smoke all passed.

Core run33786723170: Linux61 tests, macOS56, Miri5 callback +1 retained-pointer in BOTH Stacked and Tree Borrows modes, all passed. One ignored runtime test is the deliberately self-spawned fixture entry.

Runtime coverage includes natural exit/final-output ordering, exit259, strict live-captured root/PowerShell-descendant process handles, repeated termination/drop, non-reading Windows children and active-output shutdown. Combined T3 daemon coverage is tracked separately in #16.

Preserved limitations/evidence

• A pre-existing Linux blocking master-write hang after a non-reading child exits remains documented in README; no Unix I/O redesign, slave reopening, backend fork, job object or watchdog-as-cleanup claim.
• The unchanged foreground-process test hung on two old81db79a CI attempts with its background tail. Current restacked Core passed unchanged. Exact baseline/current isolated bash/dash and a500ms scheduling-gap probe did not reproduce it; logs/repro bodies remain outside Git and the issue is not dismissed or claimed fixed.
• No existing assertions or tests were weakened. T2 transport/security/Miri code and Windows API feature union were preserved in the rebase.