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.

include/plot/render/renderer.hpp

@@ -197,6 +197,11 @@ class Renderer {
     /// wrapper until the next call, which is enough for a caller measuring or
     /// laying out one glyph at a time.
     const render::Glyph* glyph(u32 codepoint) const override;
+    /// The font's cell: the atlas is the font's own, and a cell's size is a
+    /// property of the atlas, not of the size text is drawn at, so this forwards
+    /// unchanged -- which is what makes it the authoritative value the staging
+    /// path declares and the draw is told.
+    Vec2 cell_uv() const override;
     const u8* atlas_pixels() const override;
     u32 atlas_width() const override;
     u32 atlas_height() const override;