fix(pane-stream): release FIFO read fd on pane WebSocket disconnect

[tdody]

Summary

Fixes a backend file-descriptor leak that degrades responsiveness the longer Switchboard stays open — the UI gets sluggish and typing into panes lags.

Reported symptom: "As the application stays open over time, the UI becomes much slower, typing isn't as responsive."

Root cause

PaneStreamer.run() streams a pane by transferring the FIFO read fd into an asyncio read transport via connect_read_pipe, then sets the bare fd = -1. The finally block only guarded that now-dead fd and never closed the transport:

file_obj = os.fdopen(fd, "rb", buffering=0)
fd = -1  # ownership transferred
await loop.connect_read_pipe(..., file_obj)   # transport now owns the fd
...
finally:
    if fd >= 0:          # always -1 here → dead code
        os.close(fd)
    os.unlink(fifo_path) # unlinks the file, but the fd stays open

The transport only self-closes on EOF. On a WebSocket disconnect the tmux cat writer is still alive, so no EOF arrives — the .fifo is unlinked but the read fd leaks, one per terminal open. Over a working session this climbs toward the process fd soft limit (256 on macOS) and bloats the asyncio selector (it polls every dead FIFO fd each loop iteration), slowing every backend operation — including the keystroke round-trip that carries typing.

Fix

Capture the transport from connect_read_pipe and close it deterministically in finally, with file_obj / bare-fd fallbacks for the earlier failure windows. transport.close() releases the fd and deregisters the loop reader.

Verification

• New regression test test_run_releases_fifo_read_fd_on_ws_disconnect — models the exact no-EOF disconnect path (writer held open) and asserts no fd survives teardown. Fails before the fix (leaked 1 fd), passes after.
• Full backend suite: 386/386 pass. Ruff + ty clean.
• Live (before): +1 FIFO fd per open/close cycle (98 → 104 over 6 cycles).
• Live (after): 0 leaked fds across 8 clean cycles and 6 worst-case mid-stream closes; total process fds flat.

Notes

• The frontend was investigated and cleared — no JS heap leak (idle, modal churn, and heavy streaming all plateau under authoritative CDP GC). The transient heap growth during streaming is normal young-gen garbage that GC reclaims.

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