No panic from a notice wider than its band, or from the rest of its class

The showcase GUI died in joinNotice on a long shader-error notice: with
the tagline narrower than the body, the band hides more columns (skip) than
a long message has padding (pad), and `skip..@min(pad, ..)` ran backwards.
An audit of draw.zig, Layer.zig, Presentation.zig and animation.zig found
the same class elsewhere, all fixed:
- a multi-row pane tag or header scrolled for its caret's line sliced a
  shorter row past its end (paintPaneTag, renderHeaderLayer);
- a zero-width column tag kept an earlier frame's viewport, and a column
  move's clip took it below zero (now saturating, and the early return
  resets the viewport and caret);
- a tag line past 65535 columns overflowed tag_scroll's u16;
- a hover left over from a bigger window dashed past the grid;
- a border drag over a column that went away, a move preview past a
  shrunken grid, a column-move rail at zero columns;
- Layer.pointAt cast a host's unchecked float metrics (web) to u16;
- Text.insertVerticalCursor read past its lines for a cursor whose text
  changed under it.

Regressions: a test of a notice wider than its band through Msg and
through setMessage (the Post.zig compile-error path), which panics without
the fix; and a monkey over the core at a gui's metrics (random resizes to
a cell, long notices, prompts, multi-row tags, clicks, keys), 400 steps in
the suite, PARDES_FUZZ_SEED/STEPS for longer runs; 60 seeds x 3000 steps
clean.

Shared files touched: none of the list (draw.zig, Layer.zig, Text.zig).

0 files changed, 0 insertions, 0 deletions