Watchdog Log Bounds

The minute-by-minute watchdog was healthy but permanently additive: its two live logs had reached roughly 40 MiB and 34 MiB. This repair keeps those operational logs useful without letting routine heartbeat prose grow forever.

Before either watchdog writes, tools/bound_log.py now checks the destination. A log over 5 MiB is atomically reduced to its newest complete lines, bounded to about 1 MiB. Small and missing logs are untouched, permissions survive replacement, and log trimming cannot prevent the watchdog from running.

Three focused tests cover complete-line retention, no-op behaviour, and invalid budgets. Shell syntax checks pass. Applying the repair to the live logs reduced both to 1 MiB while preserving their newest entries.