fix(install): make Ctrl+C abort indexing promptly

[HumanBean17]

Problem

Ctrl+C during install (and reprocess/init) did not exit — the parent hung until the child closed its pipes, often requiring SIGQUIT/kill.

Root cause

_popen_capturing_stderr (the helper that drains cocoindex/graph subprocess output) did:

t_out.join()   # ← blocks here, uninterruptible
t_err.join()
code = proc.wait()

Thread.join() with no timeout calls lock.acquire(block=True, timeout=-1), and CPython's acquire_timed does a single blocking lock with no signal-polling loop for infinite timeout. So while the drain threads were blocked on proc.stdout/stderr.read() — which stays true as long as the child holds the pipes open — SIGINT could not interrupt the join. cocoindex's teardown (and its flow-server grandchild) can hold those pipes open for a long time, so the install couldn't be aborted.

Confirmed by reading CPython's _wait_for_tstate_lock and by a regression test that fails on the old order and passes on the fix.

Fix

Wait on the child first — Popen.wait() is interruptible (PEP 475: os.waitpid returns EINTR, CPython raises KeyboardInterrupt). On abort, terminate the child non-blocking and re-raise without joining the drain threads (a join would re-introduce the hang; daemon threads vanish at exit). The daemon drain threads keep the child's pipes empty while we wait, so it never blocks on a full pipe. On the normal path, joins are safe because the child is gone → pipes EOF → threads finish promptly.

Also catch KeyboardInterrupt in cli._console_script_main and jrag._console_script_main → flush + os._exit(130):

• clean exit (no traceback)
• routes through os._exit to skip the interpreter-finalization SIGABRT documented in that same function for commands that loaded lancedb (e.g. Ctrl+C during the optimize phase)

Tests (tests/package/test_pipeline.py)

• test_popen_captures_normal_child_output — regression guard that the reorder didn't break normal stdout/stderr/code capture.
• test_popen_abort_terminates_child_without_joining — when proc.wait() raises while drain threads block forever, the helper must terminate the child and re-raise without joining. Runs the SUT in a daemon thread with a deadline so a regression fails the assertion instead of hanging the session. Verified to fail (assert not True) on the old join-first order and pass on the fix.

Verification

• Targeted: tests/package/test_pipeline.py (6) + installer/cli/graph-only subset (123) pass.
• Full suite: 1302 passed, 14 skipped, 0 failures.
• Both entrypoints route Ctrl+C to a clean os._exit(130) (verified by simulating KeyboardInterrupt from main).

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