test: Phase 100 mutable accumulator fixture + smoke (numeric validation)

- Add apps/tests/phase100_mutable_accumulator_min.hako
  * Pattern2 with break (if i >= 3 { break })
  * Two accumulators: out = out + ch (string), count = count + 1 (integer)
  * print(count) for stable numeric output
- Add phase100_mutable_accumulator_vm.sh smoke test
  * HAKO_JOINIR_STRICT=1 validation
  * Numeric-only output extraction (expected: 3)
- Regression: all phase100/96/94 tests pass

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

apps/tests/phase100_mutable_accumulator_min.hako

@@ -0,0 +1,16 @@
+static box Main {
+    main() {
+        local out = ""
+        local count = 0
+        local i = 0
+        loop(i < 10) {
+            if i >= 3 { break }
+            local ch = "x"
+            out = out + ch
+            count = count + 1
+            i = i + 1
+        }
+        print(count)
+        return 0
+    }
+}