perf(build): the stylesheet minifier had never run

It looked for the tag written as `css` immediately followed by a
backtick. The plugin is a Rollup transform, so the module has already
been through TypeScript by the time it arrives, and the TS printer puts
a space there: `css `. The guard therefore returned null for every
stylesheet in the project, and minification never ran once — around
23 KB of explanatory comments went to every user in every release.

Matching the tag as a word with optional whitespace turns it on:

  chunk, raw       1 079 508 -> 1 021 115 B   (-58 393)
  initial view     300 111 -> 287 284 B gzip  (-12 816)
  room to the budget   955 -> 13 782 B

The ceiling moves down with the fact, as the tool asks when a graph
shrinks past the band.

The risk is not the two lines; it is that 23 KB of CSS is minified for
the first time. Two witnesses cover it: a browser smoke that puts the
original and the minified text into separate stylesheets and compares
the serialised rules — 1 049 of them, identical up to the whitespace
policy the minifier declares — and a test that takes real comment text
out of src/styles and requires it to be absent from dist, so a plugin
that silently stops working cannot pass again.

Issue: #526
User-Visible: yes

scripts/mutation-gate.mjs

@@ -3611,6 +3611,32 @@ const MUTANT_DEFINITIONS = [
       replace: "    if (ledger.caught[mutant.id] === fingerprint || mutant.id in ledger.caught) skipped.push(mutant);",
     }],
   },
+  {
+    id: 'css-minifier-skips-typescript-output',
+    guard: 'node --test --test-name-pattern="#526 AC4|#526 AC2" test/bundle-assets.test.mjs',
+    because: '#526: плагин видит вывод TypeScript, где тег отделён от шаблона пробелом; '
+      + 'строгий отсев `css` вплотную к бэктику молча выключает минификацию целиком — '
+      + 'так 23 КБ комментариев и 12,8 КБ gzip уехали пользователю и никто не заметил',
+    patches: [{
+      file: 'scripts/css-template-minifier.mjs',
+      find: "      if (!id.endsWith('.ts') || !CSS_TAG" + '.test(code)) return null;',
+      replace: "      if (!id.endsWith('.ts') || !code.includes('css" + String.fromCharCode(96) + "')) return null;",
+    }],
+  },
+  {
+    id: 'css-minifier-eats-required-space',
+    guard: 'node demo/smoke_css_minifier_semantics.mjs',
+    because: '#526: минификация 23 КБ CSS включилась впервые — свидетель обязан ловить '
+      + 'съеденный значащий пробел (потомковый комбинатор, аргумент calc), а не только '
+      + 'считать сэкономленные байты',
+    patches: [{
+      file: 'scripts/css-template-minifier.mjs',
+      find: '    if (pendingSpace && out && !TIGHT_AFTER.has(out.at(-1))' + ' && !TIGHT_BEFORE.has(char)) {
+      out += ' ';
+    }',
+      replace: '    void pendingSpace; // mutant: значащий пробел больше не восстанавливается',
+    }],
+  },
   {
     id: 'marker-shadow-animates-again',
     guard: 'node demo/smoke_marker_shadow_transitions.mjs',