an ambient meeting suppresses a nudge for its own span (V-513)

The ambient endpoint writes calendar_event_* and never calendar_busy, so a
notification-derived meeting was good enough to recite out loud and not good
enough to stop a nudge during it. Backwards: being wrong here costs one nudge.

The loop gatherer now derives busy from the event facts themselves, so the
expiry IS the meeting's span. No new level, no interval to choose, and no way
for the suppression to outlive the meeting. calendar.FactSpan reads back what
FactValue wrote; anything that does not parse says nothing about now.

cmd/mavweb/ambient.go

@@ -31,12 +31,11 @@ import (
 // not a guesser-of-truth, and a mailbox of noise rendered as invented meetings
 // is worse than a gap.
 //
-// KNOWN GAP: this writes calendar_event_* and nothing else, so an ambient
-// meeting is good enough to recite and not good enough to stop a nudge —
-// calendar_busy is still written only by the CalDAV poller. That is backwards,
-// since suppressing a nudge is the lower-risk use of a low-confidence signal.
-// calendar_busy is a level rather than an event, so an ambient writer needs an
-// expiry, which is its own task and not a change here.
+// This writes calendar_event_* and nothing else, and since Vikunja #513 that is
+// enough to stop a nudge as well as to recite: the loop gatherer reads the event
+// family and asks whether any span covers the instant. So there is no ambient
+// calendar_busy and no expiry to pick — a level needs one and an event carries
+// its own. calendar_busy stays the CalDAV poller's key.
 
 // ambientMaxBody bounds the request. A notification is two short lines.
 const ambientMaxBody = 8 << 10