2048-Style Merge Puzzle

Build a 2048-style merge puzzle game as ONE self-contained HTML file (all CSS/JS inline, no libraries, works offline).

Rules to implement exactly:
- 4x4 grid. Swipe (touch) or arrow keys move all tiles. Optionally WASD.
- Tiles with the same value merge once per move into their sum. Spawns a 2 (90%) or 4 (10%) in a random empty cell after every successful move.
- Game over when no merges are possible; win banner at 2048 with a "keep going" option.
- Score = sum of merged values; best score kept for the session.

Implementation requirements:
- Implement slide-merge as a pure function of (grid, direction) returning the new grid, merges and gained score — so it is testable. NO DOM manipulation inside that function.
- Smooth tile slide animation (CSS transforms are fine) and a small pop animation on merge.
- Tiles must be positioned with transforms so animation is cheap; 60fps target.
- Handle rapid key input gracefully (queue or throttle).
- Responsive: canvas or DOM-based is fine, must scale to phone screens.

Then print: the merge function alone, followed by a list of edge cases you handled (corner pushes, triple tiles, spawn after no-move, etc.).