The threshold came from the most recent frozen key: a frozen day in the
future, or a gap in the frozen run, pushed it past open days whose files
were then never read, and the agent would have frozen them empty for good.
It is now the oldest open day of the window (the same day as before for the
agent's contiguous frozen prefix).
Project, model and day maps are now Object.create(null): a project named
__proto__ or constructor wrote NaN counters onto Object.prototype and
dropped out of the top.
Resolves#49
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>