Regenerate the library index, which had fallen a day behind

check:library-index was FAILING: the derived menu was 70,718 seconds — about
19.6 hours — older than library/composables/useMapFly.js, which this flight
edited to give fitBounds an explicit duration. The file's own header says
"DERIVED — cannot drift from the code", which is only true of a file that gets
regenerated, and nothing had regenerated it.

The regeneration added one entry, and it is not cosmetic: viewportScrim.js was
missing from the index entirely while being imported by six files —
PosterDialog, SemanticTreeLegend, PosterWall, DataTable, LegendTools and
my-upg-100-list/poster-wall. A real, widely-used utility that an inheritor
reading the library menu could not discover. 85 reusable items now catalogued.

Measured the wider hygiene question while here, since the standing work list
asks for non-reusable components to be demoted out of the shared zone: of the 82
items the index advertises, exactly ONE has zero code importers —
useMapSelection.js. The other 81 are live. So the shared library is in good
shape and there is no demotion sweep to run.

The first measurement said fifteen components were unused, and it was wrong in a
way worth recording. It counted importers among BUNDLES only, so anything
imported by a sibling library component read as dead — PosterPreview.vue has
five importers and would have been demoted. Counting importers anywhere, not
just in plugin-designs/, is what reduced fifteen candidates to one.

useMapSelection is left in place deliberately, because removing a file from the
shared zone is a different kind of change from regenerating a derived doc and
deserves its own commit. It is carded instead. Worth knowing before that
decision: it is 192 lines, unchanged since the initial commit d5ba21f, so it was
never adopted rather than orphaned by a refactor; its one dependency
(getCircleRadiusInterpolation) still resolves, so it is dead rather than broken;
and it appears in zero of the nineteen built bundles, which is the decisive
evidence since Vite would tree-shake exactly this.

documentation/reference-library-index.md

@@ -164,6 +164,8 @@ and a change here can break other maps.
   `import … from '@map/utils/embedBase.js'`
 - **geoUtils** — geoUtils.js — Geographic / GeoJSON Utility Functions  
   `import … from '@map/utils/geoUtils.js'`
+- **viewportScrim** — viewportScrim.js — make a position:fixed full-screen SCRIM actually cover the VIEWPORT,  
+  `import … from '@map/utils/viewportScrim.js'`
 
 ### Stores (Pinia — shared state)