Add justfile, ols/odinfmt config, tests and Gitea workflows; drop odin.json

- justfile with build/run/check/test/release/package/fmt/clean recipes
- ols.json and odinfmt.json for neovim (ols)
- Tests for memory game rules and card animation; memory.deal and
  memory.select_card split out so logic is testable without a window
- Gitea workflows: build+test on main/develop, package and upload to the
  generic package registry on version branches
- Game changes to the executable's directory so packaged builds find assets

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

odinfmt.json

@@ -0,0 +1,7 @@
+{
+    "$schema": "https://raw.githubusercontent.com/DanielGavin/ols/master/misc/odinfmt.schema.json",
+    "character_width": 120,
+    "tabs": false,
+    "spaces": 4,
+    "newline_style": "LF"
+}