docs(ledger, glossary): ratify the fold summary-row timestamp semantics

Fieldtest spec_gap (#335): the timestamp a fold's one summary row
carries was undocumented. Ratified as-is (derived decision, minuted on
the issue): the row is emitted at finalize and stamped with the instant
of the last contributing (warm) value (TapFold::last_ts) — first alone
pins the first contributing instant, and min/max deliberately do not
carry the extremum's instant (a whole-window row privileges no interior
instant, and the extremum ts would cost extra state for no consumer).

The issue's initial finalize-ts reading (and the first draft of this
ratification) was refuted in review against tap_fold.rs: a tap that
goes cold before run end stamps the last WARM eval's instant, not the
run-end instant — observationally identical only while the tap stays
warm to the end, which is what the fieldtest fixtures did. Correction
minuted on the issue.

Recorded in C27's Current state and the glossary tap-plan paragraph;
the fold-registry roster doc lines state it on the surface itself
(sibling commit).

closes #335

docs/design/contracts/c27-declared-taps.md

@@ -84,11 +84,11 @@ consumer (`aura-runner::tap_recorder::TapRecorder`) holds the trace store's
 streaming writer in-graph — `initialize` opens (deferred acquisition), `eval`
 appends `(Timestamp, Cell)` (zero per-cycle heap, #77), `finalize` reports
 exactly one terminal outcome; fold consumers (`aura-std::TapFold`) land one
-summary row, stamped with the source value's own timestamp for `first`/`last`
-(they pin a single source instant) and with the finalize timestamp for the
-accumulator folds (`count`/`sum`/`mean`/`min`/`max` summarise a range and have
-no single source instant; `min`/`max` deliberately do not carry the extremum's
-timestamp — ratified as-is, #335); live closures run inline
+summary row, emitted at finalize and stamped with the instant of the last
+contributing (warm) value — `first` alone pins the first contributing
+instant; `min`/`max` deliberately do not carry the extremum's timestamp (a
+whole-window row privileges no interior instant) — ratified as-is, #335;
+live closures run inline
 (`aura-std::TapLive`). The sweep/reduce
 path never calls `bind_tap`.