fix(aggregator): attribute a midnight-straddling turn's By Activity to today on the all-provider view - #878

What

A deep audit of the recently-merged batch surfaced one real regression introduced by #853 (midnight-straddling turn slicing): on the all-provider, multi-day headline, a turn spanning local midnight lost the post-midnight half of its By Activity total and its daily turn count, even though the headline cost, calls, By Model, and By Project were all correct.

buildDurablePeriod derived today's slice of that view from the unsliced whole-range parse (src/usage-aggregator.ts), so the straddling turn kept its category and turn count anchored on its yesterday start. Call-derived cost/calls bucketed onto today correctly (per #853), but the turn-level category stayed on yesterday and the union then took today from a slice with an empty category map.

Fix

Slice the today parse with filterProjectsByDays(raw, {today}) first — the same re-anchoring the provider-filtered path already relies on. That moves the straddling turn's surviving today calls (and its category) onto today. Because day-aggregator accumulates category cost as the sum of the slice's own calls, day-N + day-N+1 still equals the whole-range total (no over-count); the per-day turn-count split matches the cache side and the documented per-day semantics.

Verification

• The bug and the fix, measured through the real cache+live union (isolated dirs, seeded daily cache): before, By Activity summed to $0.45 of a $1.35 straddling turn and today's daily row read turns: 0 with calls: 1; after, By Activity sums to $1.35 and today carries turns: 1.
• No over-count / conservation intact: 27 conservation checks (cost/calls/tokens/project-filter/watermark) all pass on the fix.
• Regression test added to the midnight-straddling turn conservation suite, mutation-checked (fails on the pre-fix code, passes with it).
• tsc clean; full CLI suite green apart from the two pre-existing parser.test.ts failures that reproduce on main.

Also

Fills in the CHANGELOG.md ## Unreleased entries that were missing for the batch (#853, #856, #872, #846/#859, #866/#867, plus a #833 citation).

Found via a post-merge audit; headline money was never wrong, so this is a By-Activity / daily-turn-count accuracy fix, not a total.