PR #30136 review noted: container-boot.log was append-only with no
rotation. On a long-lived container with frequent restarts and
many profiles it would grow unboundedly (~80 B per profile per
reconcile pass).
Add a soft cap: when the file size hits 256 KiB (`_LOG_ROTATE_BYTES`,
≈3000 reconcile lines, ≈1 year of daily reboots × 5 profiles), the
current file is renamed to `container-boot.log.1` (replacing any
existing one) before new entries are appended. Worst case is two
files at ~512 KiB — well within visibility limits for grep/cat.
Rotation is intentionally simple (no logrotate or s6-log machinery
for one append-only file). Failures during rotation are logged via
the module logger and treated as non-fatal — we keep appending to
the existing file rather than dropping the reconcile entry. Three
new unit tests cover above-threshold rotation, below-threshold
non-rotation, and overwrite of an existing .1 file.
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