chore(todo): split TODO.md into per-issue files under TODO/

Convert the flat TODO.md into a TODO/ folder where each GAL-N issue is
its own Linear-style markdown document with YAML frontmatter (id, title,
status, parent, labels), and add a README index grouped by section.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

TODO/GAL-50.md

@@ -0,0 +1,19 @@
+---
+id: GAL-50
+title: UI
+status: In Progress
+parent: null
+labels: [polish, ui]
+---
+
+# GAL-50: UI
+
+Score, lives, stage, high-score, start menu, and restart UI.
+
+## Sub-issues
+
+- [x] [GAL-51](GAL-51.md) — Display Score, Lives, and Stage in the window title.
+- [ ] [GAL-52](GAL-52.md) — Display Score, Lives, and Stage on the screen using `bevy_ui`.
+- [ ] [GAL-53](GAL-53.md) — Implement a High Score system (saving/loading).
+- [x] [GAL-54](GAL-54.md) — Create a Start Menu state with a "Start Game" button.
+- [ ] [GAL-55](GAL-55.md) — Add a "Press R to Restart" message to the `GameOver` screen and implement restart logic.