Custom block server-render: WASM↔JS bridge via hook bus (block.render:{slug}/{handler})

[tayebmokni]

Summary

Implement the WASM-JS bridge for plugin-supplied block server-renderers. When a block declares render: { handler }, the renderer pipeline (1) looks up the registering plugin from the blocks namespace, (2) constructs synthetic hook name block.render:{plugin_slug}/{handler}, (3) dispatches as a FILTER via the hook bus, (4) caches result by (block_type, attrs_hash, content_version).

The bridge has no separate WASM export — it is one more dispatch through the single hook_handler entry point.

Design reference

• docs/02-plugin-system.md §7.4 (cross-refs doc 04 §2.1, doc 07 §15.5)

Acceptance criteria

• registerBlock SDK helper constructs the canonical doc-04 BlockTypeDefinition shape (same shape as core blocks)
• Block attribute schemas validated against JSON Schema 2020-12 (per §7.7)
• When server-renderer pipeline encounters a block with render: { handler }:
  • Look up registering plugin from block namespace (gn-seo/meta -> gn-seo)
  • Build hook name block.render:gn-seo/seo_meta.render
  • Dispatch as filter via hook bus with HookCall{block: {name, attributes}, context: {post_id, view, locale, ...}}
• Plugin pre-declares the hook in its manifest hooks.filters for fast-path lookup, OR registers in init() via host_register_hook
• Renderer caches result by (block_type, attrs_hash, content_version) per doc 07 §15.5
• Cache tags emitted via the cache.invalidate capability follow doc 07 §15 conventions
• Block with render but no save -> dynamic (server-rendered every fetch unless cached)
• Block with both save and render -> static HTML for public site + server-render available for preview/admin
• Tests: server-render dispatch round-trip; cache hit on identical attrs; cache invalidation by tag

Dependencies

#73 (hook bus), #95 (hook handler), #214 (frontend SDK), doc 04 (block editor), doc 07 §15 (cache tags)

Complexity

M

[tayebmokni]

Closing — shipped in the final-wave PR (#487-#492). Auto-close only fires for first ref.