Add CI pipeline and reusable test harness

A Gitea Actions workflow (.gitea/workflows/ci.yml) running the headless test
suite on push, plus shared test scaffolding: seeded-RNG / approx / tally helpers
(test_helpers.gd), a harness self-test, and a resolution + pseudo-locale
UI-layout robustness guard.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CCyaJPTHCH9EDthpf4A5Sj

tests/harness_selftest.gd

@@ -0,0 +1,27 @@
+extends SceneTree
+## Proves tests/test_helpers.gd behaves before the rest of the suite leans on it: seeded RNGs are
+## reproducible, the approx comparisons accept tiny error and reject real gaps, and the tally maps
+## onto the exit code. It dogfoods the tally it is testing.
+## Run: godot --headless --script tests/harness_selftest.gd
+
+const TH := preload("res://tests/test_helpers.gd")
+
+
+func _init() -> void:
+	_run.call_deferred()
+
+
+func _run() -> void:
+	var t := TH.new()
+
+	var same_a := TH.make_rng(42)
+	var same_b := TH.make_rng(42)
+	t.ok(same_a.randf() == same_b.randf(), "same seed yields the same draw")
+	t.ok(TH.make_rng(42).randi() != TH.make_rng(7).randi(), "different seeds diverge")
+
+	t.ok(TH.approx(0.1 + 0.2, 0.3), "approx tolerates float rounding error")
+	t.ok(not TH.approx(1.0, 1.5, 0.1), "approx rejects a real gap")
+	t.ok(TH.vec_approx(Vector3(1, 2, 3), Vector3(1, 2, 3.00001)), "vec_approx tolerates a tiny offset")
+	t.ok(not TH.vec_approx(Vector3.ZERO, Vector3(0, 0, 1)), "vec_approx rejects a real offset")
+
+	quit(t.report())