Fix Rich Markdown embeds becoming broken links in isolated view after save

[FadhlanR]

Background and Goal

A card/file embed inserted into a RichMarkdownField previews correctly in the compose editor but renders as a broken link in the saved isolated/embedded view (CS-12111). This fixes that so a just-added embed resolves in the saved view exactly as it did in compose.

Where to start

• packages/base/query-field-support.ts — ensureQueryFieldSearchResource. The reuse branch now rebuilds an inert cached resource; provenance is recorded at creation.
• packages/base/card-api.gts — FallbackCardStore gains an isFallbackCardStore brand so the query-field layer can recognize the storeless fallback.

Key decisions and non-obvious mechanics

• Root cause: the compose editor resolves refs via a live getCards search, while the saved isolated view resolves only through the query-backed linkedCards / linkedFiles relationships. That query field's search resource is cached per field-instance and reused unconditionally. When the field is first read in edit mode, getStore returns a FallbackCardStore whose getSearchResource hands back a permanently-empty, non-updating resource (and a just-created card's refs aren't resolved yet). The isolated render then reuses that inert resource → broken link.
• Fix: record whether a resource was built against the fallback store or before its $this.* query resolved, and rebuild it once a real store / resolvable query is available. It's a one-shot transition — it never fires while still on the fallback store (no loop), and the prerender seed path is untouched.
• Verified in-app on the dev stack: edit a card with a Rich Markdown body → insert :card[...] → save → isolated now renders the embed resolved (no broken-link box). Regression test added at packages/host/tests/acceptance/markdown-embed-save-isolated-test.gts covering both card and file embeds.

🤖 Generated with Claude Code

[github-actions]

Preview deployments

• Host staging preview

• Host production preview

Host Test Results

    1 files      1 suites   2h 44m 54s ⏱️
3 597 tests 3 580 ✅ 15 💤 0 ❌ 2 🔥
3 616 runs  3 597 ✅ 15 💤 2 ❌ 2 🔥

Results for commit 073989e.

For more details on these errors, see this check.

Realm Server Test Results

    1 files      1 suites   15m 1s ⏱️
1 933 tests 1 932 ✅ 0 💤 1 ❌
2 012 runs  2 011 ✅ 0 💤 1 ❌

Results for commit 073989e.

For more details on these errors, see this check.