Report the stylesheets an entry imports that no view needs any more

missingStylesheets() names an import the views need, but nothing named
one they had stopped needing: when SealShare's last card left its views,
card.css stayed in every page until a reviewer noticed by hand.
unusedStylesheets($cssEntry) reads the entry's direct package imports
and reports each one no scanned view needs, counting what a needed
stylesheet imports itself and foundation.css always; an entry that
imports all.css is left alone. Opt-in, beside missingStylesheets()
(plan step 46).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

UPGRADE.md

@@ -170,7 +170,8 @@ the contrast toggle (`<x-theme-toggle mode="contrast">`) and the Standard motion
 - **Retargeted:** `forbidColours([...])` keeps its signature and reports a left-out role where 2.0.0
   writes one: `var(--md-sys-color-…)` in CSS or an inline `style`, an `md-ink-*` class, a
   component's `color`/`tone` prop.
-- **New:** `missingStylesheets($cssEntry)` names each `@import` the views need; a `.css` file passed
+- **New:** `missingStylesheets($cssEntry)` names each `@import` the views need, and
+  `unusedStylesheets($cssEntry)` each one they no longer do; a `.css` file passed
   to `scan()` is checked for literal values and off-scale media queries.
 
 Browser tests that assert widths switch at 640/1024/1280 now