Cover the SSR re-render mode in CI: finish the astro example + add an admin-astro job

[djay]

Problem

The server-rendered re-render mode — the bridge's findChangedUnit diff that picks the smallest changed unit and does a targeted outerHTML swap instead of a full-page innerHTML swap (packages/hydra-js/hydra.src.js, the onEditChange → POST /render → swap handler) — has zero CI coverage.

It's only invoked by pure-SSR frontends that register a server-render endpoint (astro, nextjs). Reactive frontends (mock, nuxt) never call it. And the SSR frontends have no CI-gated admin tests:

• admin-nextjs / admin-f7 projects exist but run in no CI job (opt-in only).
• There is no admin-astro project at all.

This was made concrete in #236: findChangedUnit only scanned the items region, so editing a block in a non-items region (e.g. a footer) returned { unit: 'page' } → a full-page swap that destroys the contenteditable cursor every keystroke. That bug could only be caught by a unit test (packages/hydra-js/findChangedUnitRegions.test.js) — there was no integration path to exercise it, because the one render mode that uses findChangedUnit isn't tested end-to-end anywhere.

Why we can't just run the existing admin suite on astro today

The astro example (docs/examples/test-astro, the architecture proof from #229, "commit 1/3") isn't complete enough:

• It is items-only — src/pages/api/render.ts literally types its input as blocks_layout: { items: string[] }, so it has no concept of a footer (or any multi-region) layout. (The same .items assumption we've been hunting elsewhere.)
• The renderer is generic/minimal — it doesn't handle the block types the integration suite needs (columns, grid, teaser, listing, …).

So running the admin suite against it today would fail across the board for missing-renderer reasons, not real bugs.

Scope

1. Finish the astro example (essentially feat(astro): Astro example frontend (architecture proof, commit 1/3) #229 commits 2/3, 3/3):
   • Multi-region rendering: walk blocks_layout.<region> (items, footer, …), not just items, in render.ts + [...slug].astro (mirror the test-frontend / nuxt footer-region rendering, incl. a #footer-content area).
   • Block-type coverage the integration suite exercises (slate, columns, grid, teaser, listing, …) — enough to pass tests-playwright/integration.
2. Add an admin-astro Playwright project (modelled on admin-nuxt) pointing the iframe at the astro frontend, with its webServer entry gated like the other example frontends.
3. Add a parallel CI job (test-astro-admin) running the integration suite (or a focused subset that drives editing) against astro.

Payoff

• Turns the entire SSR re-render mode from a CI blind spot into covered.
• The forced-footer columns test added in fix(helpers): region-aware nested-container merge in expandTemplates(Sync) #236 would then exercise findChangedUnit end-to-end on astro for free — no SSR-specific test needed.
• Catches the whole class of "non-items region not handled in the targeted-swap path" that fix(helpers): region-aware nested-container merge in expandTemplates(Sync) #236 had to guard with a unit test.

Context

Follow-up from the review of #236 (region-aware nested-container merge + page-level findChangedUnit). Tracks the only piece of regions-awareness we deliberately left to a unit test because no SSR integration coverage exists yet.