Add standalone test coverage for common/'s MiniJson

Requested after MCMapper-Backend's Phase 2: development follows TDD
(test-first) from here on — this retrofits the piece already built
(MiniJson, the hand-rolled JSON codec) before that request landed.

No JUnit/Gradle involved, matching common/'s "must compile standalone
under plain javac" constraint (see README.md) — a hand-rolled assertion
runner in src/test/java, compiled and run via the new run-tests.sh.
Covers write escaping, nested object/array encoding, parsing (including
the exact columns-message shape DefaultBackendConnection actually
builds), and a write-then-parse round-trip.

Deliberately not wired into either leaf's build.gradle sourceSets (which
only pull in src/main/java) — verified both forge-1_12_2 and forge-1_7_10
still compile with src/test/ present, confirming test code doesn't leak
into the shipped mod jar.

common/README.md

@@ -17,3 +17,20 @@ Minecraft/Forge/NeoForge API usage — it must compile standalone under plain `j
   request/response). Mirrors the WS protocol described in the root plan.
 - `config/` — the common config model (backend URL, server token, tracking/reconciliation
   intervals) each leaf loads via its own loader-specific config system.
+- `json/`, `ws/` — the hand-rolled JSON codec and RFC 6455 WS client `DefaultBackendConnection`
+  is built on (no third-party dependency, for the same "no shading through legacy ForgeGradle"
+  reason this module stays dependency-free generally).
+
+## Testing
+
+```
+./run-tests.sh
+```
+
+Compiles `src/main/java` + `src/test/java` and runs every `*Test.java` class's `main()`. No
+JUnit/Gradle — same "must compile standalone under plain `javac`" constraint as the module
+itself, and test code never ships in the mod jar so it doesn't need to match either leaf's JDK 8
+target. Anything needing a live Forge/Minecraft world (event hook wiring, world reads) isn't
+unit-testable this way — those stay integration-tested against a real running
+MCMapper-Backend instance instead (see the Phase 1/2 commit messages for how that's been done
+so far).

common/run-tests.sh

@@ -0,0 +1,21 @@
+#!/usr/bin/env bash
+# Compiles and runs common/'s standalone test classes (no JUnit, no Gradle — see README.md for
+# why: common/ must compile under plain javac, independent of either leaf's ForgeGradle
+# toolchain, and test code never ships in the mod jar so it doesn't need to match the leaves'
+# JDK 8 target either). Any JDK on PATH works.
+set -euo pipefail
+cd "$(dirname "$0")"
+
+OUT=$(mktemp -d)
+trap 'rm -rf "$OUT"' EXIT
+
+javac -d "$OUT" $(find src/main/java src/test/java -name '*.java')
+
+status=0
+for class in $(find src/test/java -name '*Test.java' | sed 's#src/test/java/##; s#\.java$##; s#/#.#g'); do
+  echo "== $class =="
+  java -cp "$OUT" "$class" || status=1
+  echo
+done
+
+exit $status

common/src/test/java/com/octoturge/mcmapper/common/json/MiniJsonTest.java

@@ -0,0 +1,142 @@
+package com.octoturge.mcmapper.common.json;
+
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Hand-rolled test runner (no JUnit) for {@link MiniJson}, following the same
+ * "common/ must compile and run standalone under plain javac" constraint as the class under
+ * test — see common/README.md for why, and common/run-tests.sh to actually run this.
+ */
+public class MiniJsonTest {
+    private static int passed = 0;
+    private static int failed = 0;
+
+    public static void main(String[] args) {
+        test("writeObject encodes strings, numbers, booleans, and null", MiniJsonTest::writesScalars);
+        test("writeObject escapes special characters in strings", MiniJsonTest::escapesStrings);
+        test("writeObject encodes nested objects and arrays", MiniJsonTest::writesNestedStructures);
+        test("parse reads a hello_ack-shaped object", MiniJsonTest::parsesHelloAck);
+        test("parse reads nested arrays of objects (a columns message)", MiniJsonTest::parsesColumnsMessage);
+        test("parse unescapes strings", MiniJsonTest::parseUnescapesStrings);
+        test("parse reads numbers as Double, including negatives", MiniJsonTest::parsesNumbers);
+        test("write-then-parse round-trips a columns-shaped message", MiniJsonTest::roundTripsColumnsMessage);
+
+        System.out.println();
+        System.out.println(passed + " passed, " + failed + " failed");
+        if (failed > 0) {
+            System.exit(1);
+        }
+    }
+
+    private static void writesScalars() {
+        Map<String, Object> obj = new LinkedHashMap<>();
+        obj.put("type", "hello");
+        obj.put("ok", true);
+        obj.put("count", 3.0);
+        obj.put("missing", null);
+        assertEquals("{\"type\":\"hello\",\"ok\":true,\"count\":3.0,\"missing\":null}", MiniJson.writeObject(obj));
+    }
+
+    private static void escapesStrings() {
+        Map<String, Object> obj = new LinkedHashMap<>();
+        obj.put("text", "line1\nline2\t\"quoted\"\\backslash");
+        String json = MiniJson.writeObject(obj);
+        assertEquals("{\"text\":\"line1\\nline2\\t\\\"quoted\\\"\\\\backslash\"}", json);
+    }
+
+    private static void writesNestedStructures() {
+        Map<String, Object> column = new LinkedHashMap<>();
+        column.put("x", 1.0);
+        column.put("z", 2.0);
+        List<Object> columns = new ArrayList<>();
+        columns.add(column);
+        Map<String, Object> msg = new LinkedHashMap<>();
+        msg.put("type", "columns");
+        msg.put("columns", columns);
+        assertEquals("{\"type\":\"columns\",\"columns\":[{\"x\":1.0,\"z\":2.0}]}", MiniJson.writeObject(msg));
+    }
+
+    @SuppressWarnings("unchecked")
+    private static void parsesHelloAck() {
+        Object parsed = MiniJson.parse("{\"type\":\"hello_ack\",\"ok\":true,\"serverId\":\"abc-123\"}");
+        Map<String, Object> obj = (Map<String, Object>) parsed;
+        assertEquals("hello_ack", obj.get("type"));
+        assertEquals(Boolean.TRUE, obj.get("ok"));
+        assertEquals("abc-123", obj.get("serverId"));
+    }
+
+    @SuppressWarnings("unchecked")
+    private static void parsesColumnsMessage() {
+        String json = "{\"type\":\"columns\",\"dimension\":0,\"columns\":["
+                + "{\"x\":1,\"z\":2,\"height\":64,\"blockId\":2,\"blockMeta\":0},"
+                + "{\"x\":3,\"z\":4,\"height\":65,\"blockId\":3,\"blockMeta\":0}]}";
+        Map<String, Object> obj = (Map<String, Object>) MiniJson.parse(json);
+        List<Object> columns = (List<Object>) obj.get("columns");
+        assertEquals(2, columns.size());
+        Map<String, Object> first = (Map<String, Object>) columns.get(0);
+        assertEquals(1.0, first.get("x"));
+        assertEquals(64.0, first.get("height"));
+    }
+
+    @SuppressWarnings("unchecked")
+    private static void parseUnescapesStrings() {
+        Map<String, Object> obj = (Map<String, Object>) MiniJson.parse("{\"error\":\"bad \\\"token\\\"\\nvalue\"}");
+        assertEquals("bad \"token\"\nvalue", obj.get("error"));
+    }
+
+    @SuppressWarnings("unchecked")
+    private static void parsesNumbers() {
+        Map<String, Object> obj = (Map<String, Object>) MiniJson.parse("{\"a\":-5,\"b\":3.5,\"c\":0}");
+        assertEquals(-5.0, obj.get("a"));
+        assertEquals(3.5, obj.get("b"));
+        assertEquals(0.0, obj.get("c"));
+    }
+
+    @SuppressWarnings("unchecked")
+    private static void roundTripsColumnsMessage() {
+        // Mirrors exactly what DefaultBackendConnection.sendDeltas() builds — the real
+        // regression risk isn't MiniJson in isolation, it's this shape drifting silently.
+        Map<String, Object> col = new LinkedHashMap<>();
+        col.put("x", 7.0);
+        col.put("z", -3.0);
+        col.put("height", 70.0);
+        col.put("blockId", 2.0);
+        col.put("blockMeta", 0.0);
+        List<Object> columns = new ArrayList<>();
+        columns.add(col);
+        Map<String, Object> msg = new LinkedHashMap<>();
+        msg.put("type", "columns");
+        msg.put("dimension", 0.0);
+        msg.put("columns", columns);
+
+        String json = MiniJson.writeObject(msg);
+        Map<String, Object> reparsed = (Map<String, Object>) MiniJson.parse(json);
+        assertEquals("columns", reparsed.get("type"));
+        assertEquals(0.0, reparsed.get("dimension"));
+        List<Object> reparsedColumns = (List<Object>) reparsed.get("columns");
+        Map<String, Object> reparsedCol = (Map<String, Object>) reparsedColumns.get(0);
+        assertEquals(7.0, reparsedCol.get("x"));
+        assertEquals(-3.0, reparsedCol.get("z"));
+        assertEquals(2.0, reparsedCol.get("blockId"));
+    }
+
+    private static void test(String name, Runnable body) {
+        try {
+            body.run();
+            passed++;
+            System.out.println("PASS  " + name);
+        } catch (AssertionError e) {
+            failed++;
+            System.out.println("FAIL  " + name + " — " + e.getMessage());
+        }
+    }
+
+    private static void assertEquals(Object expected, Object actual) {
+        if (expected == null ? actual != null : !expected.equals(actual)) {
+            throw new AssertionError("expected <" + expected + "> but got <" + actual + ">");
+        }
+    }
+}