Implement deferred features: in-game options menu + level editor

[kennethsinder]

Implements the two features that were deferred during the modernization (and documented in CLAUDE.md as "good next tasks"). Both are reachable from the title screen — O for options, E for the editor.

Options menu — scenes/options.py + ui/widgets.py

• A small keyboard-driven menu toolkit: Toggle / Slider / Choice / Action rows + a Menu that owns navigation and rendering. Fully unit-tested without any mouse hit-testing.
• The scene edits the live Settings and persists them on exit.
• No dead toggles: it only surfaces settings that actually do something — music, volume, fullscreen, decorations, and a developer overlay. Audio and fullscreen apply immediately; the legacy antialiasing/resolution/vsync knobs are launch-only or vestigial and stay out of the UI.
• Wires developer_mode to a real debug HUD in LevelScene (player pos/vel, lives, coin/key state, entity counts), so that toggle is meaningful.

Level editor — scenes/editor.py over world/editing.py

• A schematic, mouse-driven editor: number keys pick a tool (platform / coin / block / pool / ledge / door), left-click places, right-click erases, [/] size the brush, arrows pan, S/L save & load lvl_custom.dat (in the user data dir), P test-plays.
• The display-free EditorModel holds the records, enforces the .dat 25-slot limits, round-trips through world/loader.py, and exposes playability().
• playability() encodes the structural rules World.from_level_data needs to build a level without crashing or hanging: ≥3 platforms with ≥2 distinct non-spawn x-positions. (I traced an infinite loop in generate_mobs for degenerate levels — its placement while only terminates when a non-spawn platform's x differs from the longest platform's. The editor refuses to test-play a level that would trip it.)
• Test-play uses a new sandbox LevelScene mode (custom_data= + sandbox=True) that builds from editor data and pops back to the editor on win/death/Esc instead of routing to the title/game-over flow.

Tests (+20, 96 total)

Widget logic, options navigation + on-disk persistence, editor model add/erase/snap/limits/round-trip/validity, the title→options and title→editor transitions, the developer-overlay render path, and the full editor→play path running headless (which also exercises the generate_mobs hang guard).

Verification

uv run pytest (96 pass) · ruff check · ruff format --check · mypy src · headless smoke run — all green locally.

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