The ring buffer is bounded (2000) but the NDJSON file was append-only and grew
forever. Add size-based rotation mirroring opencode's keep-N model: track bytes
written in-process (seeded from statSync on open, so we avoid a statSync on every
write) and, when the next line would cross LOG_MAX_BYTES (5 MiB), shift
.log -> .log.1 -> ... -> .log.5 (LOG_KEEP=5, oldest dropped) and resume on a
fresh file. Rotation is best-effort and fully try/catch-wrapped: any fs failure
leaves us appending to the existing file rather than crashing logging. Adds a
temp-dir rotation test (seeds >5 MiB to force a rotation on next write).
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