Remove the per-slot dropzone wrapper from the Builder

Problem/Motivation

The Builder island renders each component for real, inside its iframe, and currently wraps the content of every slot in a display_builder:dropzone element (<db-dropzone class="db-dropzone">) to serve as the SortableJS drop container.

That wrapper is an extra box between a component and its own slot content, and it breaks the rendered layout in several ways:

• a slot rendered inside a display: grid / flex container no longer has the component template's element as the direct child, so grid/flex item semantics, gap and :nth-child() selectors stop applying;
• the wrapper's own min-width / min-height (idle, and larger while dragging or highlighting) can grow ancestors;
• because SortableJS needs that box to work, the wrapper could not simply be hidden or display: contents (SortableJS measures the container box; see SortableJS #1853).

Steps to reproduce

1. Build a page whose component renders slot content inside a grid or flex container (e.g. a hero with a grid slot, or a list rendered from a slot).
2. Open the Builder and compare the iframe rendering with the front-end output: the slot content sits one level deeper than the template intends, and the grid/flex layout of the component is wrong.
3. Enable "Drop zones" in the Highlight menu: the extra min-height/padding makes it worse.

Proposed resolution

Stop wrapping the Builder's slot content. The slot marker becomes a pair of HTML comments carrying a base64 payload (builder id, node id, slot id, title, the data-hx-post URL, max items), rendered around the slot content by a BuilderPanel::buildComponentSlot() override:

<nav class="menu"> <!-- the component's real slot container, untouched -->
<!--db-slot:eyJidWlsZGVySWQi…-->
  <li>…</li><li>…</li>  <!-- content: still the container's direct children -->
<!--/db-slot-->

A comment generates no box at all — it cannot affect grid/flex layout, :nth-child() or gap — and it cannot trip the early tag-closing that keeps dropzone.twig on a custom element. The payload is base64 so no -- sequence lands inside the comment, and it is rendered through Markup::create() because Xss::filter strips comments from plain #markup.

On the client, the builder page walks the iframe document for the markers once per refresh (annotateSlots() in builder_frame.js), decodes each payload and stamps the slot attributes onto the comment's parent — the component's real slot container — which then acts as the dropzone:

• Sortable instances live on the real containers (plus the unchanged root dropzone); insert position, locks and the cross-iframe hit test count members (direct children carrying their own data-node-id), not children, since a container may host the component's own chrome next to the slot content;
• a container hosting several slots keeps every payload; the insert position resolves to the slot whose stretch of children it falls into, and the drop POSTs to that slot's URL with an index within it;
• outline, ghost tint and slot labels apply to containers during drag/highlight states only, and contain: layout anchors each label at its own corner without touching the container's positioning.

The side panels (Scaffold cards, Navigator tree, library) keep the wrapped dropzone: their DOM is the builder's own, there is no real page layout to protect. The root dropzone is unchanged.

While testing

Selecting a component that renders as <a href=""> (a button with no configured URL) navigated the Builder iframe to its own URL — a whole reload, scroll lost, annotations gone. The click handler in builder_frame.js skipped preventDefault() for an empty href; it now keeps every link from navigating except pure # anchors. This is independent of the wrapper change and can be split into its own issue if preferred.

Remaining tasks

• Full @base Playwright run on a configured harness (the one Builder-scoped selector is updated in this patch; Navigator/library/tree specs keep the wrapped dropzone and are untouched).
• Follow-up: an automated test for the multi-slot container routing.

User interface changes

The Builder's editing canvas now renders components exactly as the front end does (only HTML comments and data-* attributes added at runtime). Highlight/drag chrome on slot containers appears only while dragging or with the "Drop zones" highlight on. No changes outside the Builder.

API changes

None. RealRenderTrait::buildComponentSlot() changes visibility from private to protected so the Builder panel can override it; the default (wrapped) implementation is unchanged for other panels.

Data model changes

None. Stored instance data is untouched; the markers exist only in the Builder's rendered output.

Testing steps

1. Build an entity view / page layout containing components with grid and flex slots (e.g. dro-hero, feature grids).
2. Open the Builder: the rendering matches the front-end output — inspect the iframe DOM to confirm slot content are direct children of the slot container, with <!--db-slot:…--> markers around them.
3. Drag components between slots and reorder within a slot; verify insertion position survives the round trip.
4. Enable "Drop zones": each slot shows its outline and label at its own top-left corner; empty slots give the pointer somewhere to land.
5. Click a component that renders as an empty link: the Builder must not navigate or scroll.