refactor(admin): remove circular deps and recover lazy HMR loads

[DavidBabinec]

Summary

• Reduced the tsconfig-aware source graph from 31 circular dependencies to 0 by extracting pure leaf schema/type/helper barrels and moving cycle-sensitive internals off broad barrels.
• Added retryable lazy/HMR loading for the Site editor body: failed prewarmed imports clear their failed state, expose reset(), and render a retryable chunk boundary instead of leaving the admin on an endless skeleton.
• Added architecture coverage for zero circular dependencies and tightened the Fast Refresh/lazy-body gates.

Impact

The broad app-facing barrels remain available, while cycle-sensitive internals now depend on pure leaves such as @core/page-tree-schema, @core/module-engine-schema, @core/site-runtime-schema, @core/visual-components-schema, and @core/html-sanitize. Base module editor components no longer import their registration barrels, which keeps Fast Refresh boundaries component-only.

Verification

• bun x madge --circular --ts-config tsconfig.json --extensions ts,tsx src server — processed 2282 files, 3 warnings, 0 circular dependencies.
• bun test src/__tests__/architecture/no-circular-dependencies.test.ts — 1 pass, 0 fail.
• bun test src/__tests__/architecture/canvasFastRefreshBoundaries.test.ts src/__tests__/architecture/site-editor-shell-lazy-body.test.ts — 8 pass, 0 fail.
• bun run build — passed.
• bun test — 5486 pass, 0 fail.
• bun run lint — passed.

Manual Smoke

• Ran local disposable dev stack on PORT=3102, VITE_PORT=5175, VITE_ALLOWED_ORIGIN=http://localhost:5175, DATABASE_URL=sqlite:./.tmp/e2e-hmr.db, UPLOADS_DIR=./.tmp/e2e-hmr-uploads.
• Opened /admin/site, completed local setup, loaded the Site editor body past the skeleton, added a base Container module, temporarily edited src/modules/base/container/ContainerEditor.tsx, and observed Vite HMR update the canvas placeholder without falling back to an endless skeleton. The temporary probe edit was reverted before commit.