fix(test): out-of-process watchdog for the httpapi exerciser - #177

2026-08-05 CI incident: the effect-mode run froze for 13 minutes with zero output after "worktree.create: shared use done" until the 15m step timeout. The scenario timeout and the bounded() cleanup guards are all timer-based — they cannot fire when a native-level hang (instance dispose / tree-sitter / sqlite teardown) freezes the event loop itself, which is the class of failure the 2026-07-27 hardening did not cover.

The only guard that survives a frozen event loop is a separate process:
- the runner heartbeats a file on every scenario/phase transition (--progress mode, i.e. CI)
- a child process polls it every 5s; after 120s of silence it prints the last recorded scenario/phase and SIGKILLs the runner
- a silent 15-minute freeze becomes a 2-minute attributed failure; the step timeout stays as the final backstop

Verified: worktree scenario subset passes with the watchdog armed; a synthetic event-loop freeze is killed in ~4s (3s timeout + poll) with the last-activity diagnostic, exit 137.