fix(tui): reap PTY child on startup failure

[lawrencecchen]

Summary

• keep local PTY children owned through fallible startup setup
• kill and wait when setup or reaper-thread creation fails
• avoid a second kill after a successful wait

Verification

• rustfmt and git diff --check
• hosted cmux-tui verification follows after PR creation

This change is limited to local PTY startup in cmux-tui-core/src/surface.rs; terminal-host runtime is unchanged.

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Summary by cubic

Fixes a PTY child process leak where a spawned child stayed alive if local terminal startup failed after spawn. Terminal-host runtime is unchanged.

• A startup guard now kills and reaps the child on any failure path and tracks whether the wait already reaped it to avoid a second kill against a reused PID.
• Reaper-thread creation failure now closes the ConPTY master before returning so reader and frame threads can release the surface.

^{Written for commit 50ce113. Summary will update on new commits.}

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Project	Deployment	Actions	Updated
cmux166	Ready	Preview	Sep 2, 2026 3:08pm UTC
cmux41	Ready	Preview	Sep 2, 2026 3:08pm UTC

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📥 Commits

Reviewing files that changed from the base of the PR and between 0247f51 and 50ce113.

📒 Files selected for processing (2)

• cmux-tui/crates/cmux-tui-core/src/surface.rs
• cmux-tui/crates/cmux-tui-core/src/terminal_host_protocol.rs

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