fix: #3620603 Seed the backing page with the node's default full-content layout

Fixes a defect I introduced in the previous MR on this issue. The page-backed fallback 500s on any content type whose ContentTemplate binds to the entity — which is every real template — and it did not carry the default layout over.

Issue: #3620603

The bug

The backing canvas_page was seeded from the node's default ContentTemplate verbatim. A template's props are bound to the entity it renders, and a canvas_page has no node to bind them to, so the save throws:

EntityStorageException: `entity:node:blog[title][value]` is an expression for entity type
`node`, but the provided entity is of type `canvas_page`.

On Varbase 11's blog template, 6 props are entity-bound.

Why the previous MR missed it: it was verified on a Drupal CMS site whose page bundle had no ContentTemplate at all, so the seeding branch never ran against a real template, and the sync was proven with a tree hand-copied from another canvas_page — which by definition holds no node-bound expressions. The plumbing was proven; the realistic case was not.

The fix

Overriding must start from the content type's default full-content layout — that is what makes it an override rather than a blank slate. So the layout is carried over in full, with each entity-bound prop evaluated against the node and stored as its value:

$item['inputs'][$prop] = PropSource::parse($value)->evaluate($node, FALSE)->value;

A prop that cannot be evaluated is dropped so the component falls back to its own default — losing one prop beats losing the layout, and leaving the expression in place would make the save throw.

Because the layout is now per-node, those values are a snapshot taken at override time rather than live bindings. On the patched path the loader still seeds the node's own field, where the bindings stay live. That difference is documented on the class.

Two alternatives were rejected: seeding nothing (an empty editor defeats the feature), and filtering the bound components out (on this template that leaves 6 of 11, so the editor would open on a layout silently different from the template).

Verified on a fresh Varbase 11.0.x site (Drupal 11.4.6, canvas 1.10.1) through the browser

Unpatched (final class, guard active, fallback path):

Check	Before this MR	After
Fallback editor	500 EntityStorageException	opens on the default layout: breadcrumb, node title as heading, date, Share block, header and footer
Seeded page tree	not reached	11 items — matches the template's 11
entity:node expressions on the page	not reached	none
Page save to node field	not reached	11 items synced
Node renders the layout	not reached	7,532 bytes
drush cr	works	works

Patched (varbase-patches applied, loader swap active) — unchanged by this MR:

Check	Result
Content type toggle	enabled, no warning
First save of the setting	works: field storage and instance both created
Editor /canvas/editor/node/15	loads, template layout renders
Reset flow (HTMX + CSRF)	204 + HX-Redirect

phpcs: 0 errors.

AI-Generated: Yes

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