feat(transition): add corner_reveal, a rectangular uncover anchored at a corner

[LeadcodeDev]

What

Reproducing a reference piece turned up a scene transition none of the thirteen existing types can express. Measured frame by frame, over 15 frames (0.5 s):

	right	top	left	bottom
f3 → f18	pinned 3832	pinned 0	2160 → 0	1480 → 2152

Two edges stay on the frame, two travel, and both moving edges advance at once. The incoming scene sits still behind the growing window: it is uncovered, not pushed.

wipe_* moves one full-width band along a single axis; iris is a circle; slide translates both frames together. A dissolve was the closest available approximation and it loses the gesture entirely.

API

{ "type": "corner_reveal", "duration": 0.5, "corner": "top_right",
  "easing": "ease_in_out" }

corner selects which two edges are pinned — top_right (the measured default), top_left, bottom_right, bottom_left — and is inert for every other type.

Verified against the measurement

Rendered at 1920×1080, the same law comes back:

f24  left 1512  right 1916  top 0  bottom  228
f30  left  688  right 1916  top 0  bottom  692
f36  left    0  right 1916  top 0  bottom 1076

Tests

Five, pinning the geometry rather than describing it:

• the anchored edges never move, at any progress
• the travelling edges open monotonically, in the direction the corner names
• empty at 0, exactly the full frame at 1
• each corner anchors its own pair — otherwise corner is decoration
• out-of-range progress clamps instead of inverting the rectangle: an inverted rect clips to nothing, and the transition would silently render as a cut

cargo test --workspace green, cargo fmt --check and cargo clippy --all-targets -- -D warnings clean.