/metrics: forge_process_restarts_total and forge_child_processes{comm} #3727

Split from #3708, waits for #3690 (GET /metrics).

#3708's pull counts starts in boots.json (src/kernel/boots.ts: bootCounts() gives boots, restarts, unclean) and reads the process table by name (src/kernel/census.ts: census(processRows())). Today both are on log lines (forge is up, the autonomy tick, the minute processes line).

Done:

1. /metrics exposes forge_process_restarts_total (counter, bootCounts().restarts) and forge_process_unclean_restarts_total (unclean).
2. /metrics exposes forge_child_processes{comm="socat"} and so on (gauge, one series per name from census), label values escaped per OpenMetrics.
3. A test that boots the app and reads both series.

Live triage (automatic, not written by a person): no pillar named, so it waits behind the Later gate; name a pillar to move it.