[codex] Add described-by filter chip

[rdhyee]

Summary

• Add a pinned side-panel chip for active described-by concept filters, rendered from window.__searchFilter with the label escaped via escapeHtml().
• Wire the chip clear button through the existing search-filter clear path and remove described-by from the URL.
• Hide the cluster placeholder while any search/concept filter is active and restore it after clearing.

Validation

• quarto render explorer.qmd
• Served docs/ on port 8079 and headlessly loaded ?described-by=https://w3id.org/isample/vocabulary/materialsampleobjecttype/1.0/wholeorganism; asserted the chip shows Whole organism material sample plus ✕, #clusterSection is hidden, and clicking ✕ clears the filter plus URL parameter.
• git diff --check

Fixes #255

[rdhyee]

Superseded by #261, which shipped the consolidated runtime (stats-collapse #254 + chip #255 + facet-count fix #253) to isamples.org. Closing the PR. NOTE: the regression tests from these branches are not yet on upstream — they'll be re-added via one clean follow-up PR (incl. repackaging the count-race spec as a standalone .mjs to fix the [P2] default-suite//data dependency).