refactor(render): the atlas is a declared fixed-cell grid, and staging checks it

The text pipeline stages a glyph's rect *origin* per instance and carries the
rect's *size* once per draw, so a backend whose glyphs have differing rect sizes
-- a TrueType packing trimmed to ink, which is Task 8's shape -- would have drawn
every glyph at whichever size it happened to be told, with nothing downstream
able to tell. Make that constraint structural rather than documented:

- TextBackend::cell_uv() declares the one cell size, and the contract (an atlas
  is a fixed-cell grid; a proportional face draws its glyphs *into* a uniform
  cell instead of trimming each rect to its ink, and why the pipeline needs it)
  is stated on the atlas accessors.
- glyph_is_cell() is the check; add_text()/set_cell() declare the cell from the
  backend, and the staging path asserts it for every glyph in a debug build.
  A release build draws on with the cell it was told, as before.
- Renderer::flush_text() takes u_glyph_uv from the backend, not from the batch,
  so the draw's one rect size can never come from whichever glyph was staged
  first.

tests/test_gl_context.cpp

@@ -384,7 +384,7 @@ PLOT_TEST(text_pipeline_draws_a_glyph) {
   program.bind();
   program.set_viewport({static_cast<f32>(ctx.width), static_cast<f32>(ctx.height)});
   program.set_px_range({static_cast<f32>(BitmapFont::kGlyphPx), static_cast<f32>(BitmapFont::kGlyphPx)});
-  program.set_glyph_uv(batch.glyph_cell_uv());
+  program.set_glyph_uv(font.cell_uv());
   program.set_texture_size(atlas.texel_size());
   program.set_color(plot::Color::hex(0xFF00FF));
   atlas.bind();