watch: signal handling and termination semantics #34

From the UX audit: no signal handlers, SIGTERM kills with 143, bounded exits always 0 whether --count or --timeout stopped the loop, and the jsonl fallback can write a raw non-JSON line into the stream (Sources/aps/Aps.swift:148-157).

Acceptance criteria

• SIGINT/SIGTERM stop the loop cleanly (DispatchSourceSignal), exit 0 on SIGINT after final flush
• Bounded exit reason is observable: terminal jsonl event or stderr line with reason count|timeout|signal
• jsonl stream never contains non-JSON lines
• Tests cover termination paths; specs updated

Locked decisions (design interview round 2, 2026-07-18)

• Termination: both channels. Terminal JSONL event {type:end, reason:count|timeout|signal} in machine mode and a stderr line in human mode; exit codes 0 (count) / 124 (timeout, GNU convention) / 130 (SIGINT). Bare unbounded watch prints a one-time stderr hint suggesting --count/--timeout.