dispose() ended stdin and sent SIGTERM in the same tick, so the child's
EOF-driven quiesce had no window to run. The real acp-agent has no SIGTERM
handler in a normal session — it flushes persistence and stops child-owned
work via the server bridge's connection-close path (conn.closed → per-agent
dispose → final session/flush), driven by stdin EOF, NOT by a signal. A prompt
response can resolve from a turn/end before that post-turn flush lands, so the
child still owes durable work when dispose runs; a same-tick default SIGTERM
terminated it mid-flush, orphaning child-owned bash and dropping the flush.
dispose now waits for the child's natural exit after stdin EOF first, then
escalates SIGTERM (grace), then SIGKILL — a three-tier ladder. Add an
`exitsWithin` helper for the bounded waits.
Regression coverage: a new mock mode (MOCK_FLUSH_ON_EOF) flushes a marker
asynchronously on EOF then self-exits; the tier-1 test asserts the marker
lands (proven RED on the same-tick-SIGTERM ordering — child killed mid-flush).
MOCK_IGNORE_EOF covers the middle tier (ignores EOF, dies on default SIGTERM);
the existing MOCK_TRAP_SIGTERM test covers the SIGKILL tier.
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