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.
