fix(canvas): make table & structural containers droppable; empty table cells fill the row

[avanelsas]

Problem

Several BareDOM container components are CSS-grid based and collapse to ~0 height when empty on the canvas, so in edit mode they had no footprint to drop children into and couldn't be authored. x-table was the worst case:

• With no columns set it fell back to implicit max-content tracks (overflow) and its subgrid rows collapsed to a single stacked column.
• Empty x-table-cells picked up the generic empty-container affordance's margin: 4px 0, and since a stretched grid item fills track − margins, empty cells rendered ~8px shorter than and vertically offset from their populated siblings — looking broken even once columns was correct.

Changes

• meta/slots.cljs — register slot metadata for x-table / x-table-row / x-table-cell and a batch of other structural containers (x-bento-grid, x-fieldset, x-form, x-collapse, x-tabs, x-carousel, x-timeline(+item), x-scroll*, …) that previously fell through to the leaf default and got no container affordance.
• meta/augment.cljs + ui/palette.cljs — x-table's columns is a grid-template-columns track list its subgrid rows inherit. Drive the inspector field through the :grid-columns transform (edit a count, store a real track list) and seed a dropped table with repeat(3, 1fr) — same convention as x-grid — so a fresh table has a real, editable column config rather than relying on the auto-flow fallback.
• render/canvas_view.cljs — arm canvas-pan from the composed-path target, not .-target. Inspector fields are custom elements (x-search-field, x-text-area) whose real input lives in shadow DOM; a keydown retargeted to the host made the editable-widget check miss and swallowed spaces typed into inspector fields. Extracted editable-target? as a pure, tested fn.
• public/index.html (editor-only CSS, .canvas-host):
  • table-fallback tracks when columns is unset, and min-width: 0 so populated table elements shrink to their track;
  • empty x-table-cell override (margin: 0; min-width: 0) so empty cells stretch to the row height and follow their column track, while the min-height floor + dashed (empty) label keep an all-empty row droppable.

Tests added for slot metadata, the columns transform/default, and the space-pan editable-target guard.

Verification (all four gates green)

• clj-kondo --lint src test scripts — 0 errors, 0 warnings
• cljfmt check — clean
• npx shadow-cljs compile test — 866 tests / 2532 assertions, 0 failures, 0 errors
• npx shadow-cljs release app — build completed, 0 warnings

Note: the genuine width unevenness in x-table itself (integer columns shorthand expanding to repeat(N,1fr) rather than minmax(0,1fr)) is fixed separately in the BareDOM library — avanelsas/baredom#265. This PR is the editor-side companion.

🤖 Generated with Claude Code

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Also includes a small unrelated cleanup: manifest-url's docstring (export/integrity.cljs) wrapped before @vanelsas/baredom, making Closure parse @vanelsas as a JSDoc tag and emit a parse note on every release build. Rewrapped so @ follows the /; release build is now note-free.