router: introduce typed ActionValidationStatus boundary (slice 5)

Introduce ActionValidationStatus enum (valid, unresolved, missing_argument,
invalid_argument, ambiguous_target) as the typed classification of validation
outcomes. ActionValidationResult now carries Status instead of boolean flags.

Backward-compatible: Unresolved() and Valid() methods preserved on the result.
Existing validation behavior unchanged: only blank Fn produces invalid_argument.
All downstream behavior (proposeGap, confirmation, task_status, praxis, hexis)
unchanged.

Tests added for all five status values, backward compatibility, and the full
validation → execution boundary.

cmd/mavend/actionresolve.go

@@ -54,22 +54,23 @@ func noteActionResolution(ctx context.Context, source, fn string, resolved bool)
 }
 
 // noteActionValidation records the structural validation outcome in the
-// decision trace. Three outcomes: unresolved (matcher miss), valid
-// (structurally admissible), or invalid (structurally malformed).
+// decision trace. Five outcomes: unresolved (matcher miss), valid
+// (structurally admissible), invalid_argument, missing_argument, or
+// ambiguous_target (structurally malformed).
 func noteActionValidation(ctx context.Context, v router.ActionValidationResult) {
 	rec := decision.From(ctx)
 	if rec == nil {
 		return
 	}
-	switch {
-	case v.Unresolved:
+	switch v.Status {
+	case router.ActionUnresolved:
 		rec.Note(decision.Claim{
 			Stage:    decision.StageAction,
 			Claimant: "action-validation",
 			Outcome:  decision.Declined,
 			Reason:   "unresolved",
 		})
-	case v.Valid:
+	case router.ActionValid:
 		rec.Note(decision.Claim{
 			Stage:    decision.StageAction,
 			Claimant: "action-validation",
@@ -77,9 +78,9 @@ func noteActionValidation(ctx context.Context, v router.ActionValidationResult)
 			Reason:   "valid",
 		})
 	default:
-		reason := "invalid"
+		reason := string(v.Status)
 		if len(v.Issues) > 0 {
-			reason = "invalid:" + v.Issues[0].Reason
+			reason = string(v.Status) + ":" + v.Issues[0].Reason
 		}
 		rec.Note(decision.Claim{
 			Stage:    decision.StageAction,