Structure level numbers: classic bitmap font by default + graphics toggle (#4264)

## What

Structure **level numbers** now render in the **`round_6x6_modified`**
bitmap font by default (matching the old PIXI-based `StructureLayer` /
`v31`), with a graphics setting to switch back to the smooth
`overpass-bold` MSDF font.

Two commits:

1. **Default to the classic bitmap font** — `StructureLevelPass` drew
level digits from the `overpass-bold` MSDF atlas (the one `NamePass`
uses for player names); switch the default to the `round_6x6_modified`
pixel font (white digits with a baked-in dark outline).
2. **Add a runtime toggle** — load both fonts and switch between them
live via a new `Classic level numbers` graphics setting.

## How

- `StructureLevelPass` loads both atlases up front and selects one per
frame from `settings.structureLevel.classicFont`, re-laying-out the
digits when the toggle flips (digit advances differ between the fonts).
The fragment shader is a single program with a `uClassic` branch: direct
bitmap sample (white fill + baked outline) vs. MSDF median + synthesized
outline.
- New override `structure.classicNumbers` in `GraphicsOverrides`
(default `true` = classic), applied onto
`settings.structureLevel.classicFont` in `applyGraphicsOverrides` — so
it switches live, like the existing colorblind/classic-icons toggles.
- `GraphicsSettingsModal` gets a `Classic level numbers` toggle next to
`Classic icons` (with `en.json` strings).

## Testing

- `tsc --noEmit`, ESLint, Prettier, and `npm run build-prod` all pass.
- Ran the game headless, built/upgraded cities to level 2–3, and
confirmed: the classic toggle renders the pixel font, flipping it
renders the smooth MSDF font, and flipping back restores the pixel font
— switching live with no shader errors.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/client/render/gl/render-settings.json

@@ -155,7 +155,8 @@
   "structureLevel": {
     "scale": 1,
     "outlineWidth": 8,
-    "offsetY": -0.4
+    "offsetY": -0.4,
+    "classicFont": true
   },
   "bar": {
     "healthBarW": 11,