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/bitmap_font.hpp

@@ -40,6 +40,13 @@ class BitmapFont final : public TextBackend {
   static constexpr u32 kAtlasRows = 7;
   static constexpr u32 kAtlasWidth = kAtlasCols * kGlyphPx;    ///< 128 texels
   static constexpr u32 kAtlasHeight = kAtlasRows * kGlyphPx;   ///< 56 texels
+
+  /// The atlas cell, normalised: one `kGlyphPx` square of the grid. Every glyph's
+  /// rect is this, and `cell_uv()` reports it -- one number, derived from the same
+  /// two constants `pack_atlas()` packs, so the grid the pipeline draws and the
+  /// grid the ink was written into cannot disagree.
+  static constexpr Vec2 kCellUV{static_cast<f32>(kGlyphPx) / static_cast<f32>(kAtlasWidth),
+                                static_cast<f32>(kGlyphPx) / static_cast<f32>(kAtlasHeight)};
   static_assert(kAtlasCols * kAtlasRows >= kGlyphCount, "atlas too small for the glyph set");
 
   BitmapFont() noexcept = default;
@@ -71,6 +78,9 @@ class BitmapFont final : public TextBackend {
   /// what a label outside the coverage set should show.
   const Glyph* glyph(u32 codepoint) const override;
 
+  /// `kCellUV`: the 8x8 cell every glyph occupies.
+  Vec2 cell_uv() const override { return kCellUV; }
+
   /// The packed atlas, as `TextBackend` sees it: the bytes `atlas_image()`
   /// returns, `kAtlasWidth` by `kAtlasHeight`, GL_R8-ready. Never null.
   const u8* atlas_pixels() const override;