Space the letters of a text object closer or wider apart

The Feuerwerk wordmark is set with -0.03em tracking. Text in the
engine had no letter spacing, so the wordmark came out about 3 % wider
than the logo.

- Text has a new property Letter spacing, default 0, in the same unit
  as the font size. It goes between each two characters, so ten
  characters get it nine times. Negative values pull them together.
- TextPicture hands the value to Pixi's text style. Both rasters, the
  smooth and the sharp one, use it, and it is part of the raster key.
- The estimated text width in bounds.ts adds the spacing too, so
  selection, picking and "Add collider" fit the drawn text.
- Like every number property it can be animated and set from a graph.
  That is not tried in the browser.

Tests:
- New unit test for the footprint with spacing. 102 tests in the
  three text files green.
- e2e 30f-ii measures the width of "Handgloves" on screen: 237 without
  spacing, 282 at +5 (expected +45) and 210 at -3 (expected -27).
  e2e 30 on melektaus-atx: ALL PASSED, 25 checks.
- Gate on melektaus-atx: CHECK EXIT 0, 307 files, 4757 tests.
- No counter-check run for 30f-ii.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -47,7 +47,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2065 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 2066 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs