feat: add shape cues as a new stimulus channel

[manuelmauro]

Summary

Adds shape as a 4th n-back stimulus channel. The tile now renders as a geometric shape (circle, triangle, square, pentagon, or hexagon) using 2D meshes instead of a plain sprite.

How it works

When shape cues are enabled in settings, the tile changes shape each round. The player must identify when the current shape matches the one N steps back, just like with position, color, and sound.

Rendering change

The tile switched from Sprite to Mesh2d + MeshMaterial2d<ColorMaterial>:

• Shape is controlled by swapping the Mesh2d handle (from a TileMeshes resource with pre-built handles)
• Color is controlled by mutating the ColorMaterial asset

New/modified files

File	Change
game/tile/shape.rs	New — TileShape enum, random distribution
game/tile/mod.rs	TileMeshes resource, tile_shape_system, tile_color_system updated for materials, Tile requires TileShape
game/session/engine.rs	shapes channel, Cue struct replaces tuple return
game/session/answer.rs	shape field
game/session/mod.rs	Shape evaluation + cue generation in round system
game/settings.rs	shape toggle (off by default)
game/mod.rs	Mesh-based tile spawn, TileMeshes resource init
game/ui/button.rs	SameShape action
game/ui/mod.rs	Shape button (D key)
menu/ui.rs	Shape checkbox in cue grid
menu/checkbox.rs	Shape action
debug.rs	Shape match + answer display

Keyboard shortcuts (reshuffled to home row)

Key	Action
A	Position
S	Color
D	Shape
F	Sound