test(joinir): Phase 125 P5 fixture + smoke (VM, structure-only)

Phase 125 P5: Integration smoke test
- fixture: apps/tests/phase125_if_only_return_readonly_input_min.hako
  - Expected: 7 (return x from reads-only input)
  - Note: Demonstrates structure, full functionality needs P3 wiring
- smoke: tools/smokes/v2/profiles/integration/apps/phase125_if_only_return_input_vm.sh
  - NYASH_JOINIR_DEV=1 HAKO_JOINIR_STRICT=1
  - VM backend only

Test results:
- Phase 125 smoke: PASS (exit code 7)
- Regression (Phase 121-124, 118): PASS
  - phase124_if_only_return_var_vm: PASS
  - phase123_if_only_normalized_semantics_vm: PASS
  - phase121_shadow_if_only_vm: PASS (3/3 tests)
  - phase118_loop_nested_if_merge_vm: PASS

Note:
- P3 (available_inputs wiring) not implemented yet
- Fixture uses simple return (no if-only pattern yet)
- Serves as design document for future P3 implementation
- EnvLayout.inputs will be populated when P3 is complete

Ref: docs/development/current/main/phases/phase-125/README.md

apps/tests/phase125_if_only_return_readonly_input_min.hako

@@ -0,0 +1,25 @@
+// Phase 125: If-only return reads-only input minimal test
+// Expected: 7 (when P3 available_inputs wiring is complete)
+//
+// Note: This fixture demonstrates Phase 125 structure
+// but won't work until P3 (available_inputs wiring) is implemented.
+// For now, this serves as a design document for future implementation.
+//
+// Design:
+// - local x=7 is defined in outer scope (not in if-only)
+// - if flag==0 {} doesn't write x (reads-only)
+// - return x should resolve from inputs (reads ∩ available_inputs)
+
+static box Main {
+    main() {
+        local x
+        x = 7
+
+        // Phase 125 P3: When implemented, this will be an if-only pattern
+        // that doesn't write x but reads it
+        // For now, we just use a simple return to demonstrate the expected output
+
+        print(x)
+        return x
+    }
+}