Two small cleanups in the Metal text path on top of the buffer-offset
fix from cc71d95:
- Drop the SDF-style `smoothstep(0.5 ± ew, alpha)` from the text mode
branch in UI_MSL_SRC. The glyph atlas stores alpha coverage straight
from stbtt_MakeGlyphBitmap, not signed distance, so the smoothstep
was thinning anti-aliased strokes by mapping mid-coverage values
(0.3–0.7) toward 0/1. Use the sampled value directly as alpha.
- Drop the 16-byte alignment pad on `mtl_buf_offset` in `flush()`. Each
batch's upload_size is already a multiple of UI_VERTEX_BYTES (48), so
the running offset stays vertex-aligned without the extra rounding.
- After `font.shape_text` + `font.flush` in `render_text`, re-bind
`font.texture_id`. If the atlas grew during shaping, the GPU texture
handle changed; without this rebind the next flush samples the old
(smaller) atlas which doesn't have the newly-rasterized glyphs.
- Use explicit s64-pointer arithmetic in `metal_update_buffer_at_ios`
so a future regression in `[*]u8` indexing can't quietly miscompile
the per-flush write offset.
Text at small sizes still renders dim on dark backgrounds — most glyph
pixels sit in 0.1–0.5 coverage and the linear blend doesn't push them
to bright values — tracked separately as the faint-text follow-up.
b43472e6ab