Add Wave 1 block server schemas (page builder step 3, server half)

Register all seven Wave 1 block types with their server-side validation
schemas, self-registering via server/src/blocks/types/*:
heading, rich_text, image, two_column (container), cta, divider, quote.

- propHelpers.js: shared validators (isSafeUrl rejects javascript:/data:/
  protocol-relative, enum/required/optional text, strict key allowlist).
- rich_text carries a `sanitize` normalizer (registry now supports it) that
  runs html through the shared cleanBody allowlist on save.
- Registry entrypoint requires the type modules so all schemas load.

Verified: all 7 register; valid blocks pass; malformed props yield precise
per-path errors; one-level nesting cap enforced; rich_text sanitize strips
script/onerror.

Client renderers + editors (step 3 client half) still to come.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/blocks/index.js

@@ -4,22 +4,21 @@
 // page's blocks or look up a block type should require THIS module, not
 // ./registry directly, so the definitions are guaranteed to be loaded.
 //
-// Wave 1 block definitions are registered below, one require() per block, as
-// they are built (spec build order step 3). Until then the registry is empty and
-// validateBlocks rejects any block type — which is correct: no page can save a
-// block that has no server-side schema yet.
+// Wave 1 block definitions are registered below, one require() per block (each
+// module self-registers on load). Requiring THIS module guarantees they are all
+// present before anything validates a page's blocks.
 
 const registry = require('./registry')
 const { validateBlocks, MAX_BLOCKS, MAX_SUBBLOCKS } = require('./validateBlocks')
 
-// ── Wave 1 block definitions ──────────────────────────────────────────
-// require('./types/heading').register(registry)   // added in step 3
-// require('./types/richText').register(registry)
-// require('./types/image').register(registry)
-// require('./types/twoColumn').register(registry)
-// require('./types/cta').register(registry)
-// require('./types/divider').register(registry)
-// require('./types/quote').register(registry)
+// ── Wave 1 block definitions (self-register on require) ────────────────
+require('./types/heading')
+require('./types/richText')
+require('./types/image')
+require('./types/twoColumn')
+require('./types/cta')
+require('./types/divider')
+require('./types/quote')
 
 module.exports = {
   ...registry,