explorer: #248 described-by — add a removable concept chip + hide stale 'Click a cluster' placeholder

[rdhyee]

Why

described-by=<uri> has no UI surface (URL-only) and its results render below the fold. Two small fixes:

1. A pinned 'Described by: ✕' chip at the top of the side panel — visible concept identity + one-click clear. (This is the UI element users will look for; also helps because the concept filter is currently fragile — any search-box entry silently replaces it.)
2. Hide the '#clusterSection "Click a cluster or sample" placeholder' while a search/concept is active — it's meaningless then and eats the vertical space the results want.

Scope

• explorer.qmd: doDescribedBy() already sets window.__searchFilter = {kind:'concept', uri, term:<label>, ...} — render the chip from that; clear = call the existing clear path + remove described-by from the URL.
• The chip should also show for a future paste-a-URI input (Flavor B). Escape the label with escapeHtml (see XSS fix in explorer: support described-by=<concept-uri> search (#248 Flavor A) #252).
• Hide #clusterSection when searchIsActive().

Acceptance

• With a concept active, a chip shows the label + ✕ that clears it; placeholder hidden during search; Codex-reviewed; staging-verified.

[rdhyee]

Implemented for #255 and pushed as a draft PR on the origin fork: rdhyee#6

What changed:

• Added a pinned side-panel chip for active described-by concept filters: Described by: <label> ✕.
• Rendered the chip from window.__searchFilter when kind === 'concept' && active, escaping the label with the existing escapeHtml() helper before innerHTML.
• Wired the chip clear button through the existing clear path: clearSearchFilter() + applySearchFilterChange() + URL rewrite via writeQueryState({ commitText: '' }).
• Hid #clusterSection whenever searchIsActive() is true, and restored it after clearing.
• Kept doSearch()'s hot path untouched.

Validation:

• quarto render explorer.qmd
• Served docs/ on port 8079.
• Headlessly loaded ?described-by=https://w3id.org/isample/vocabulary/materialsampleobjecttype/1.0/wholeorganism and asserted the chip showed Whole organism material sample plus ✕, the cluster placeholder was hidden, and clicking ✕ cleared both the filter and the URL param.
• git diff --check

Note: direct cross-repo PR/comment creation through the GitHub integration was blocked by permissions, so the branch is currently at rdhyee:feat/described-by-chip-255 with the fork draft PR above.

[rdhyee]

Follow-up: I corrected the branch base to isamplesorg:main and opened the actual upstream draft PR:

#259

The upstream PR now shows 1 commit / 1 changed file (explorer.qmd) instead of the earlier fork-only comparison noise. Re-ran validation after the rebase:

• quarto render explorer.qmd
• Headless load of the described-by=...wholeorganism URL on port 8079, including chip display, hidden #clusterSection, and clear-button URL/filter reset
• git diff --check

[rdhyee]

Shipped to isamples.org via #261 (described-by concept chip + hidden stale placeholder during search/concept).