Migrates hyperscript demo/reference pages from grouped index files into
one-per-page directory layout. Each gallery-<topic>/index.sx is a single
defpage with its own demo, matching the one-per-file convention used
elsewhere in sx/sx/applications/.
Covers: control (call/go/if/log/repeat/settle), dom (add/append/empty/
focus/hide/measure/morph/put/remove/reset/scroll/set/show/swap/take/
toggle), events (asyncError/bootstrap/dialog/fetch/halt/init/on/pick/
send/socket/tell/wait/when), expressions (asExpression/attributeRef/
closest/collectionExpressions/comparisonOperator/default/in/increment/
logicalOperator/mathOperator/no/objectLiteral/queryRef/select/splitJoin),
language (askAnswer/assignableElements/component/cookies/def/dom-scope/
evalStatically/js/parser/relativePositionalExpression/scoping), and
reactivity (bind/live/liveTemplate/reactive-properties/resize/transition).
Adds _islands/hs-test-card.sx — a shared island for hyperscript demos.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>