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/actions_act.go

@@ -39,7 +39,7 @@ func (h *reactiveHandler) actionAct(ctx context.Context, dec router.Decision) st
 	validation := router.ValidateActionCandidate(candidate)
 	noteActionValidation(ctx, validation)
 
-	if !validation.Unresolved && !validation.Valid {
+	if !validation.Unresolved() && !validation.Valid() {
 		// Resolved but structurally malformed: refuse execution.
 		return phraser.A(phraser.ActFail, nil)
 	}