Fix RVM evaluation of default-only rules (#664)

Default-only rules (e.g., `default deny := true` with no conditional body)
returned Undefined in the RVM instead of the default value.

Compiler:
- compute_rule_type: return Complete when rule exists only in default_rules map
- compile_worklist_rule: emit register slots and data-tree entries for
  default-only rules (else branch)

VM:
- execute_call_rule_common + execute_call_rule_suspendable: check
  default_literal_index before returning Undefined when definitions is empty

Tests:
- 3 new RVM cases (default_rules.yaml): bool, object, entry-point
- 3 new interpreter cases (default/basic.yaml): matching coverage

Co-authored-by: Mark Birger <markbirger@microsoft.com>

tests/interpreter/cases/default/basic.yaml

@@ -321,6 +321,37 @@ cases:
     skip: true
     want_result: [5]
 
+  - note: default_only_rule_bool
+    data: {}
+    modules:
+      - |
+        package test
+        default deny := true
+    query: data.test.deny
+    want_result: true
+
+  - note: default_only_rule_object
+    data: {}
+    modules:
+      - |
+        package test
+        default deny := {"result": false, "reasons": []}
+    query: data.test.deny
+    want_result:
+      result: false
+      reasons: []
+
+  - note: default_only_rule_with_package_query
+    data: {}
+    modules:
+      - |
+        package graph.mypolicy
+        default deny := {"result": false, "reasons": []}
+    query: data.graph.mypolicy.deny
+    want_result:
+      result: false
+      reasons: []
+
   - note: valid-var-in-some-in-value
     data: {}
     modules: