Add REST preload paths for query loop blocks in the editor

[roborourke]

Summary

• Hooks block_editor_rest_api_preload_paths to parse post_content for core/query blocks with parse_blocks()
• For each non-inherited loop, derives the exact REST URL core/post-template's getEntityRecords() will request (mirrors per_page/offset/order/orderby/sticky shape from Gutenberg's post-template/edit.js)
• apiFetch's preloading middleware then serves first-paint posts from cache — no HTTP round-trip per loop
• Skips loops with taxQuery (taxonomy rest_base mapping is non-trivial for v1) and excludeDisplayed (depends on runtime render order)
• Capped at 10 loops per post to avoid payload bloat
• Zero JS changes — pure PHP hook

Context

This is Track B in a two-track exploration of reducing editor API load on magazine-style pages with many Query Loop blocks. See also PR #11 (Track A: explicit Preview/Edit toolbar toggle using ServerSideRender).

How to choose between the two approaches after both land:

	Track A (SSR toolbar)	Track B (preload paths, this PR)
First-paint REST requests	Still fires on initial load (editor=default, preview=on-demand)	Zero for preloaded loops
UX surface	Toolbar toggle visible on every loop	Invisible — transparent optimization
Editing experience	Preview mode → stable SSR HTML, no refetch storms	Native editor throughout
Attribute change	Re-snap when toggling back to preview	Cache miss → normal REST request
LOC / complexity	~90 lines JS	~130 lines PHP
Gotchas	Doesn't help unless user puts loops in Preview mode	One-shot cache consumption; misses loops with taxQuery

If preload gives 80%+ of the win with no UX surface area, ship this PR and close #11.

Test plan

• Start npm run wp-env start, run npm run build
• Create a page with 3+ non-inherited Query Loop blocks, publish, reopen in editor
• Open Network tab: /wp/v2/posts requests on editor load should be 0 (served from preload)
• Change a loop's perPage → editor re-fetches for that loop (expected cache miss)
• Verify page source contains wp-api-fetch inline script with preloaded REST responses
• npm run test:e2e — all existing tests pass + new preload-paths.spec.js test

🤖 Generated with Claude Code

[github-actions]

Playwright test results

 2 failed
 16 passed

Details

 18 tests across 6 suites
 1 minute, 9 seconds
 459fdd4

Failed tests

chromium › preload-paths.spec.js › Query Loop REST Preloading › should preload /wp/v2/posts REST path for query loop blocks
chromium › preload-paths.spec.js › Query Loop REST Preloading › should still fetch posts after query attributes change