Add view transitions to the shell

[AllTerrainDeveloper]

Animates the moments the shell changes state, using the browser's View Transitions API.

A transition is CSS, not JavaScript — the player starts one carrying a view-transition type, and assets/css/view-transitions.css owns every keyframe. A plugin ships one as a registry entry plus a stylesheet, with no animation code and no teardown.

What's in it

Two settings (OpenStation Preferences → Effects), one registry partitioned on a def's scope:

• Screen transitions — 25, for switching Space and for appearance changes.
• Window transitions — 7, for a window opening, closing, minimizing, restoring or maximizing. On open the window morphs out of whatever you clicked, inferred from the last pointerdown rather than threaded through every caller — so it works for launchers that don't exist yet, including a plugin's (data-os-vt-launcher).

Cross-document transitions are separate and always on: chromeless.css opts framed admin pages in, so navigating inside a window animates instead of flashing blank.

Both settings default to none, and every wired surface keeps its original un-animated code path behind a hasActiveViewTransition() guard.

How to test

Turn something on in Preferences → Effects. Picking a transition plays it immediately; "Play it again" replays.

1. Windows — set Window transitions to Morph, then click a wallpaper icon or a dock tile. The icon should stretch into the window. Then close it, minimize it, click its dock tile to bring it back, and double-click a title bar to maximize. Try Genie, Slam, Unfold for different character.
2. Desktops — set Screen transitions to Cube, then press Left / Right to change Space. Direction-aware ones should mirror between the two.
3. Appearance — set Screen transitions to Ripple, then swap the wallpaper (or the accent, or a desktop theme). The new look should flood out from where you clicked.
4. In-window navigation — open Posts in a window and click around (Posts → Add New → back). Independent of the settings above.
5. Reduced motion — turn it on at the OS level; everything becomes instant, including fix: resolve 3 PHPUnit failures + 1 error on trunk test code #4.

Three deliberate non-animations, so they don't read as bugs: session restore at boot, keyboard/programmatic opens (no click to morph from), and prefers-reduced-motion.

Chrome/Edge 126+ for the full path; older engines degrade a step at a time rather than breaking.

🤖 Generated with Claude Code