Declared PDFs: author-supplied render components via the render slot

[lukemelia]

Closes CS-12980. Stacked on #6184 (CS-12979) — review/merge that first; this PR's base is the CS-12979 branch, so its diff shows only the render-slot work.

What this does

A declared type: 'pdf' entry can now source a capture-only render component instead of a display format — the same render-slot pattern posters (PdfPosterCapture) already use — for cards whose document shape isn't any display format:

static screenshots = {
  statement: { format: 'isolated', type: 'pdf' }, // isolated-template path (CS-12979)
  invoice:   { render: InvoiceDocument, type: 'pdf' }, // custom-component path (this PR)
};

The component renders the full document flow itself and paginates via print CSS (@page, break-*); there is no capture box.

Changes

• base/card-api.gts — assertValidScreenshotSpec stops refusing render on a PDF entry. A PDF entry now sources render or an isolated/embedded format; it still refuses raster geometry, box formats, and useAsThumbnail.
• realm-server/prerender/utils.ts — render-based PDF entries drive the render.screenshot route through captureRenderBasedEntry under emulated print media (no box); page.pdf() paginates the component's flow. The PDF loop dispatches render- vs format-based sources; both stay under the PDF sub-cap and follow the broken-links model on failure.
• host render.screenshot route + template — a PDF render entry carries no width/height and renders the component in the natural document flow, not the fixed-size overflow: hidden box the raster path uses (which would clip a multi-page PDF to one page). Readiness stays the component's data-screenshot-pending signal.
• experiments-realm — a StatementPdfDemo pattern card demonstrating both paths (isolated-template + custom-component) plus an instance.

Doorway check: @cardstack/boxel-host/lib/* needs no new registration — an author PDF component loads through the same route + component loader PdfPosterCapture already uses for its pdfjs-loader import.

Acceptance (CS-12980)

• invoice: { type: 'pdf', render: InvoiceDocument } produces a multi-page PDF at index time whose content comes from the component, not the isolated template (the end-to-end test asserts the component's 3-page flow vs the template's 1 page)
• a slot-level component error follows the broken-links model without failing the row

Tests

• Unit — validation now accepts render + type: 'pdf'; refusal cases updated.
• End-to-end indexing — a render component paginates its own three-page document (distinct page count from the one-page isolated template, proving provenance) and serves as application/pdf; a failing PDF component records a screenshotErrors entry and persists no ledger row while the row still indexes.

Verification

realm-server passes lint:types + lint:js; host passes lint:js and lint:types for all files I touched (two pre-existing origin/main errors remain in app/lib/pdfjs.ts and app/lib/signed-capture.gts — unrelated to this change, from local build state). ⚠️ The realm-server browser suites (Docker-seeded Postgres + prerender stack) were not run locally — Docker wouldn't start in my environment. They need a CI run.

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Host Test Results

    1 files      1 suites   2h 15m 48s ⏱️
4 887 tests 4 873 ✅ 14 💤 0 ❌
4 902 runs  4 888 ✅ 14 💤 0 ❌

Results for commit 160a18c.

Realm Server Test Results

    1 files    241 suites   1h 17m 51s ⏱️
3 513 tests 3 513 ✅ 0 💤 0 ❌
3 561 runs  3 561 ✅ 0 💤 0 ❌

Results for commit 160a18c.