test: Phase 99 add trailing backslash escape best-effort (VM+LLVM)

末尾バックスラッシュ処理:
- 現行仕様: best-effort（そのまま出力）として固定
- fixture: "hello\\" → "hello\" を出力
- VM+LLVM EXE parity完全対応

Integration smokeで検証済み

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

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

apps/tests/phase99_json_loader_escape_trailing_backslash_min.hako

@@ -0,0 +1,38 @@
+// Phase 99: json_loader escape with trailing backslash
+// Purpose: Exercise escape loop with incomplete escape sequence (trailing \).
+//          This fixture documents current behavior: trailing \ is silently ignored.
+
+static box Main {
+    main() {
+        // Input: "hello\" - string ending with backslash (incomplete escape)
+        local s = "\"hello\\\\"
+        local pos = 0
+        local i = pos
+
+        // Skip opening quote
+        if s.substring(i, i + 1) != "\"" { return "NG" }
+        i = i + 1
+
+        local out = ""
+        local n = s.length()
+
+        loop(i < n) {
+            local ch = s.substring(i, i + 1)
+            if ch == "\"" { break }
+
+            if ch == "\\" {
+                i = i + 1
+                if i < n {
+                    ch = s.substring(i, i + 1)
+                }
+            }
+
+            out = out + ch
+            i = i + 1
+        }
+
+        // Current behavior: trailing backslash is included in output
+        print(out)
+        return "OK"
+    }
+}