test: Phase 100 string accumulator fixture+smokes (VM+LLVM)

- Add phase100_string_accumulator_min.hako fixture
  * out = out + ch string accumulation
  * print(out.length()) for stable numeric output
- Add VM smoke: phase100_string_accumulator_vm.sh
- Add LLVM EXE smoke: phase100_string_accumulator_llvm_exe.sh (Phase 97 gating)
- Regression: all phase100/97/94 tests pass

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

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

apps/tests/phase100_string_accumulator_min.hako

@@ -0,0 +1,15 @@
+static box Main {
+    main() {
+        local out = ""
+        local s = "abc"
+        local i = 0
+        loop(i < 3) {
+            if i >= 3 { break }
+            local ch = s.substring(i, i + 1)
+            out = out + ch
+            i = i + 1
+        }
+        print(out.length())
+        return 0
+    }
+}