Let a preset actually be saved

Review found the headline feature unreachable. `valid` still demanded an end
date unconditionally, while every sibling rule had been gated on
`datedWindow`. A fresh form starts with endKnown true and endDate empty, so
picking weekly hid both the field and the checkbox and left Save disabled
with nothing on screen a reader could do about it. Editing an existing event
was fine, which is why no test caught it: every static render drives the form
through `initial`, and cadenceOf never returns a preset for an event that has
an end.

So the three states that needed a click to reach are now pure functions that
do not. endStated pins exactly that case. repeatOf takes the cadence and
answers for all five, which also makes `until` surviving the preset path
provable — mutating it away previously left the suite green.

Two more from the same review. The unknown-end note keyed off the cadence, so
custom with the repeat set to never promised a countdown that would never
arrive; it keys off the rule now, which is what actually decides. And a delay
whose start and end disagree about having a time of day can never land a
whole unit from the anchor — unstatable, not merely unstated — so it says so
rather than disabling Save in silence.

"State it myself" was also a one-way door: nothing ever cleared it, so the
measured reading could not be recovered without abandoning the form.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

docs/PRD.md

@@ -331,7 +331,7 @@ decides which dates are even questions:
 |---|---|---|
 | one-off | start, end, or "I don't know when it ends" | no rule |
 | daily / weekly / monthly | a start, and nothing else | no end, a one-unit cycle |
-| custom | start, end, and how it repeats | whatever the reader states |
+| custom | start, an end it still allows to be unknown, and how it repeats | whatever the reader states |
 
 **A preset carries no window.** Choosing weekly says the week *is* the window —
 each occurrence runs until the next opens — so there is no end date to type and