json-native: token positions (line/column); escape utils BMP coverage + surrogate guard; add smokes for string escapes, nested, and error cases (AST/VM)

tools/smokes/v2/profiles/quick/core/json_string_escapes_ast.sh

@@ -0,0 +1,63 @@
+#!/bin/bash
+# json_string_escapes_ast.sh - JSON string escapes roundtrip via AST using
+
+source "$(dirname "$0")/../../../lib/test_runner.sh"
+require_env || exit 2
+preflight_plugins || exit 2
+
+TEST_DIR="/tmp/json_string_escapes_ast_$$"
+mkdir -p "$TEST_DIR"
+cd "$TEST_DIR"
+
+cat > nyash.toml << EOF
+[using.json_native]
+path = "$NYASH_ROOT/apps/lib/json_native/"
+main = "parser/parser.nyash"
+
+[using.aliases]
+json = "json_native"
+EOF
+
+cat > driver.nyash << 'EOF'
+using json_native as JsonParserModule
+
+static box Main {
+  main() {
+    // input → expected stringify
+    local inputs = new ArrayBox()
+    local expect = new ArrayBox()
+
+    inputs.push("\"A\"");            expect.push("\"A\"")
+    inputs.push("\"\\n\" ");        expect.push("\"\\n\"")
+    inputs.push("\"\\t\"");         expect.push("\"\\t\"")
+    inputs.push("\"\\\\\"");        expect.push("\"\\\\\"")
+    inputs.push("\"\\\"\"");        expect.push("\"\\\"\"")
+    inputs.push("\"\\u0041\"");      expect.push("\"A\"")
+
+    local i = 0
+    loop(i < inputs.length()) {
+      local s = inputs.get(i)
+      local r = JsonParserModule.roundtrip_test(s)
+      print(r)
+      i = i + 1
+    }
+    return 0
+  }
+}
+EOF
+
+expected=$(cat << 'TXT'
+"A"
+"\n"
+"\t"
+"\\"
+"\""
+"A"
+TXT
+)
+
+output=$("$NYASH_BIN" --backend vm driver.nyash 2>&1 | filter_noise)
+compare_outputs "$expected" "$output" "json_string_escapes_ast"
+
+cd /
+rm -rf "$TEST_DIR"