normalize_token_usage_state only length-checked persisted day keys, so a
hand-edited or foreign 10-char key (e.g. "not-a-dat3" or "2026-13-01") in
token-usage.json survived reads and atomic rewrites. token_usage_payload
then parsed every day key with an unguarded datetime.fromisoformat, so one
such key failed every /api/settings and /api/settings/usage request until
the file was repaired by hand.
Validate day keys in normalize_token_usage_state, the shared boundary that
every read, record, and rewrite already funnels through. Malformed keys are
dropped like other malformed rows and scrubbed from the file on the next
write; valid state is unchanged.
Bind SessionManager saves to the existing raw archive path so SDK imports and other bypass saves cannot persist more than the file cap without archiving unconsolidated overflow.
Add an SDK regression test that exercises the real ingest path.
Refs #4787
Keep only 128 recently used sessions strongly cached while retaining weak references to evicted sessions still owned by active callers. This bounds idle memory growth without allowing duplicate live Session objects or skipping shutdown flushes.
Add LRU, lifecycle, SDK, and flush regression coverage.
Refs #4786
KDB