feat(BlockEditor): add inline formatting with rich content model

- migrate block content from plain strings to InlineNode[] structure
- add inline toolbar (bold, italic, code, color, link) on text selection
- add checklist block type with toggle support
- add image block type (URL-based, phase 2)
- add inline serialization helpers (inlineToDom, domToInline)
- add inline types and length utilities
- extend caret utils with range get/set support
- update block registry and all existing block types for new content model
- update demo blocks in ComponentsPage to use rich inline content
- update README to reflect new architecture

src/shared/components/BlockEditor/index.js

@@ -19,3 +19,19 @@ export {
   DEFAULT_BLOCK_TYPE,
 } from './blockRegistry.js';
 export { newBlockId } from './utils/ids.js';
+export {
+  INLINE_COLORS,
+  INLINE_COLOR_KEYS,
+  inlineLength,
+  inlineToPlainText,
+  inlineFromText,
+  sliceInline,
+  concatInline,
+  applyMark,
+  removeMark,
+  toggleMark,
+  marksAtOffset,
+  marksInRange,
+  normalize as normalizeInline,
+} from './inline/types.js';
+export { inlineToDom, domToInline } from './inline/serialize.js';