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

docs/DEVELOPMENT.md

@@ -279,6 +279,26 @@ node scripts/bundle-tree.mjs dist custom_components/houseplan/frontend
 
   The full HA pytest harness requires Linux/WSL; native Windows lacks `fcntl`.
 
+## The stylesheet minifier sees TypeScript output, not the source (#526)
+
+`scripts/css-template-minifier.mjs` runs as a Rollup transform, and by then the
+module has already been through TypeScript. The TS printer puts a space between
+a tag and its template, so the source `css`…`` arrives as `css `…``.
+
+The plugin used to look for the exact string `css` + backtick and therefore
+returned `null` for every stylesheet in the project: minification never ran
+once, and roughly 23 KB of explanatory comments were shipped to every user —
+12.8 KB gzipped in the initial chunk.
+
+Two consequences for anyone touching this area:
+
+- match the tag as a word followed by optional whitespace, never as a literal
+  two-character string;
+- the guard that keeps this honest is not inside the plugin but in
+  `test/bundle-assets.test.mjs`: it takes real comment text out of
+  `src/styles/*.ts` and asserts none of it appears in `dist/**`. A plugin that
+  silently stops working cannot pass it.
+
 ## Do not animate container-relative properties on the plan (#524)
 
 A CSS property whose value is expressed in container query units — `cqw`,