Find the grid cells under layers once a frame, not by scanning every layer for each cell

renderFrame skips the grid cells a body or tag layer paints itself, and it
asked which those are cell by cell: for every cell, twice a frame (counting
instances, then emitting them), bodyLayerAt and tagLayerIn walked all 16 body
layers and all 119 tag layers. In a ReleaseFast profile of a full screen of
src/pardes.zig those walks were about a third of the gui's CPU samples.

coverLayers now fills each live layer's rectangle into a per-cell map once a
frame, and the loops read one byte. The loops only ever asked whether some
layer covers a cell, never which, so overlap order does not enter into it; the
map marks exactly the cells the walks found: a tag layer only its viewport's
first row, and a viewport past the grid only the grid's part of it. The loops
over the layers a panel transition leaves get a second map, built only while
one runs.

A test holds the map to the old walk over a core's layouts (columns, stacked
and collapsed panes, column tags, Tagbottom, message and leader chips) and
3000 random layer sets, some past the grid's edges or at the top of u16.
Hidden captures of 27 layouts (message and prompt chips, a column grip rail,
a PDF, an image, font and tagline changes), with and without leftover edge
pixels, are byte-identical to before.

Over 10 interleaved rounds (160x50 cells), renderFrame's CPU a frame falls
from about 2.0 ms to 0.32-0.37 ms idle, scrolling, typing, under terminal
spew and over a PDF (-82 to -84%, every round), and cell emission from about
1.07 ms to 0.27 ms. The first frame's CPU goes from 6.2 to 4.6 ms.

Co-Authored-By: Claude Opus 5.5 (1M context) <[email protected]>

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