Persist the dates the form always collected and always threw away

The creation form has always offered real date and time pickers, and the write
kept only the label derived from them. The user chose a date; the broker never
saw it. Five fields went that way, on create and on update alike, because the
shape defined nowhere for them to land.

`startDate`, `endDate`, `startTime` and `endTime` are now on the Event shape and
carried end to end. `date` is untouched -- it is the label the lists display, and
nothing derives one from the other. `themes` is deliberately left out: it is a
repeated value, so it needs a cardinality decision rather than one more optional
string, and nothing consumes it.

Proven on the running application, both paths, asserted only after a reconnect
because the optimistic overlay shows the values before anything is durable:
written 2026-12-05 / 09:15, read back identical; edited to 2027-01-22 / 08:05,
read back identical. An event missing the fields renders clean -- the time line
simply absent, nothing thrown.

The recorded generator drift no longer reproduces. Regenerating from the
UNCHANGED shape produced bytes identical to the committed bindings, which is how
the additive diff was separated from the generator's own; that separation was the
point of running it twice, and it turned a warning into a measurement. The
doctrine still carries the old warning and is now wrong about it.

Also worth having: the form composes its label from the ISO values joined, not
from the French wording the doctrine gives as its example.

.project/concepts/tech-stack/_debt.md

@@ -0,0 +1,7 @@
+# Doc-debt — tech-stack
+
+> Presence of a block = doc to update. Processed → delete the block; no blocks left → delete this file.
+> One block = one "big change": `why` + `files` + `verify` (leaves to review).
+
+## Raw markers (consolidate into blocks, then delete)
+- TOUCHED package.json @2026-08-17 (session 0b064e8b-1717-421f-a20e-a4318ad217b1)