Contain window-reveal play failures and stop over-promising owner

[epeicher]

Follow-up to #459, fixing review findings that surfaced after it merged.

Play failures could strand a window under the armed surface

Element.animate() throws on an easing it cannot parse — at play time, after the opaque armed surface is already covering the window and before any finish/cancel handler is attached. The exception escaped playWindowReveal into the WINDOW_CONTENT_LOADED action, so the window stayed covered on every load, the loading-overlay teardown never ran, and the remaining subscribers for that event were cut off. The custom-render path already guarded its third-party callback; the clip-path path now gets the same containment:

• Registration rejects an unparsable easing. A throwaway KeyframeEffect runs the same parser animate() uses, so the bad string dies with a stack frame at registration time. Environments without the Web Animations API accept everything — they never call animate().
• playWindowReveal survives a throwing animate() (a def injected through the desktop-mode.window-reveals filter never went through registration) by dropping that layer, and it now clears the --revealing pin when every layer bailed.
• The play-time def lookup survives a throwing filter callback by degrading to "no reveal": layers removed, content uncovered.

owner was documented as something it does not do

The reveal def table, the example, and the registerWindowReveal docblock all said owner gets a reveal live-unregistered on plugin deactivation. unregisterWindowRevealsByOwner() exists, but nothing calls it on the deactivation path — reveals have no PHP script-registration companion yet, the same known gap as palettes that the reference already acknowledged a paragraph later. The docs now describe owner as a grouping tag that starts working the moment the sweep lands, rather than promising a sweep that does not exist. Wiring the actual sweep (payload key + src/reveals/server-sync.ts, per the established pattern) is left as a follow-up since it adds PHP API surface.

Stale docblocks from the default flip

The commit that defaulted reveals to off missed two docblocks still claiming 'sweep' as the shipped default (src/settings/registry.ts, src/settings/types.ts — the file the JS reference names as the authoritative snapshot shape) and two more claiming "five built-ins" (there are twelve).

Tests

• Registration rejects a non-string easing and an unparsable easing (stubbed KeyframeEffect), and still accepts valid ones.
• A throwing animate() uncovers the window and clears the --revealing pin instead of propagating.
• A throwing reveals filter at play time uncovers the window instead of stranding it.

All suites green: eslint, tsc, vitest (2748), PHPUnit (1700).