Add four classic screensavers and group wallpapers into collections

[vaprdev]

Four new Classics wallpapers, and collections in the menu.

New wallpapers

• maze: the 3D Maze screensaver, raycast brick corridors with eased turns and pauses (dev/check.ts plays it for 2 minutes, since a 16 s sample lands on either a glide or a pause).
• pipes: 3D Pipes growing through a grid, with the teapot easter egg; only growing tips redraw.
• mystify: full-screen polygons bouncing off the edges, with stacked echoes gliding every frame.
• flying-opencode: Flying Windows with the OpenCode mark.

Collections

• Wallpaper.collection (Landscapes, Akira, Classics) and fixedTime. The menu gets a Collection row; the Wallpaper row and shuffle stay inside it. /wallpaper collection <name>.
• Akira and Classics have one fixed look: time of day, season and weather apply to Landscapes only, and their rows are hidden for the others.
• README gallery grouped by collection; fixed-look wallpapers have one screenshot.
• src/canvas.ts gains rebake() (pipes) and hold() (maze).

Images already pushed to assets.

Verified: typecheck, build, dev/check.ts --quiet --max-ms=60 in octant and quadrant modes (123 runs each, 0 failed), --verify-dirty on samples, screenshot check (41 up to date), every README image exists on assets, actionlint; the menu was driven in a real OpenCode profile.