Extend glyph render constraints with cell-span sizing modes for height,
width, contain, cover bounds, and stretch bounds. These preserve the
existing face-targeted behavior for platform fonts, emoji, and Nerd Font
rules while giving registered glyphs a target based on terminal cell
spans.
Map Glyph Protocol registration options to the new constraint modes so
sizing follows the spec formulas based on authored advance width and line
height. Baseline alignment now places design-space y=0 on the terminal
text baseline instead of approximating it as start alignment.
Document the placement formulas in the local protocol summary and add
focused tests for constraint mapping, cell-span padding, line-height and
advance scaling, contain versus cover behavior, stretch, and baseline
placement.
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