kitty fits an image to its cell rect preserving aspect, so a frame whose pixel
size isn't a whole multiple of the cell rounds up — clipping the bottom row
("clipped feet") and letterboxing a blank row. Trim each frame to its union
alpha bbox, then snap to an exact cell multiple before transmit so the sprite
hugs its box and renders full-body. (ratatui-image#57: render in multiples of
the font-size.)
Add the shared pet engine under agent/pet/: spritesheet manifest loading
and in-process caching, six-state animation model, frame rendering, and
the persistent pet store. Register the display.pet config block (pet,
scale, enabled, etc.) that every surface reads from. Covered by
tests/agent/test_pet_engine.py.
