entity_facts takes a before instant

[WaylandYang]

Closes #416, the follow-up to #373.

changes prints each event's exact record instant; the prompt then asked the model to call entity_facts with as_of one microsecond earlier, doing the fractional-second arithmetic itself — with a borrow across the second when the fraction is zero. A slip there returns the state after the correction, silently, which is the #351 failure with the arithmetic moved into the model.

entity_facts now takes before: the event's timestamp, pasted as printed. The server reads the base as it stood strictly before that change. The implementation is one line, because the ledger's clock is microseconds: strictly before T is at-or-before T − 1 µs, so held_at from 0019 is unchanged — recorded_at <= T−1µs is exactly recorded_at < T, and invalidated_at > T−1µs is exactly invalidated_at >= T. The summary line says as recorded before <T> with the instant the person asked about, not the shifted one. before overrides as_of.

The changes description and the system prompt now say: copy the timestamp into before; never compute an earlier as_of. The at description also stops saying YYYY-MM-DD only, since #414 made it accept any precision.

Tests: the microsecond shift including the borrow across a whole second, and the summary wording. Floating-Y's same-second test from #373 already pins that as_of = T − 1µs returns the retracted row and as_of = T the correction.

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