Give a suppressed rule a durable semantic identity (V-687)

The digest needs to know whether a candidate is already pending before it pays
the phraser, and prose is not identity: phrasing varies, and State.Now advancing
does not turn the same unmet condition into a new event.

A rule eligible for the digest declares DigestIdentity beside its predicate.
DigestCandidateFingerprint frames the rule name and severity around it so two
rules cannot alias on a shared fact. BreakRule anchors on the last completed
break, not on desk_active, which the poller refreshes without the unmet need
changing. A rule that declares no identity does not enter the digest, since a
generic state hash would either change every tick or ignore an input the rule
reads.

internal/loop/rules.go

@@ -23,6 +23,13 @@ type Rule struct {
 	Cooldown  // base cooldown + bounded-duration envelope for the auto-tuner
 	Predicate func(State) bool
 
+	// DigestIdentity names one semantic occurrence while this rule is held by
+	// quiet hours, presence, or calendar restraint. The durable digest checks
+	// its fingerprint before asking the phraser for prose, so it must change
+	// when the candidate's meaning changes and remain stable while only Now
+	// advances. Only rules eligible for that digest need to declare it.
+	DigestIdentity DigestIdentity
+
 	// InertWhenNoData — most rules should be silent when their substrate key is
 	// missing (since(key)==null → don't fire). If the predicate already encodes
 	// that check itself, leave this empty. Otherwise set to the key(s) the rule
@@ -109,6 +116,17 @@ func BreakRule() Rule {
 		Severity:        Sev2,
 		Cooldown:        Cooldown{Base: 45 * time.Minute, Min: 20 * time.Minute, Max: 4 * time.Hour},
 		InertWhenNoData: []string{"desk_active", "break"},
+		// The last completed break is the semantic anchor for this stretch.
+		// desk_active is freshness evidence for the predicate and may be
+		// refreshed every poll; treating each refresh as a new occurrence would
+		// defeat durable dedupe even though the unmet need did not change.
+		DigestIdentity: func(s State) []byte {
+			lastBreak, ok := s.Fact("break")
+			if !ok {
+				return nil
+			}
+			return factDigestIdentity(lastBreak)
+		},
 		Predicate: func(s State) bool {
 			dDesk, ok1 := s.Since("desk_active")
 			dBreak, ok2 := s.Since("break")