@W-22849705 feat(content): support Page Designer content blocks (fragments)

[clavery]

Summary

Adds first-class support for Page Designer content blocks — reusable, shared fragment.*-typed content — across the SDK, CLI, and VS Code extension. Previously the SDK bucketed these as components, so they were invisible as a distinct, shared concept.

This work item was a title-only stub ("Support 'Content Blocks' in Content Topic and Page Designer tree views"); the data model was established by exploring a live instance (zzpq-023 MarketStreet site library).

What a content block is (verified against the instance)

• A <content> element typed fragment.* (e.g. fragment.Content.contentCard, fragment.Layout.grid) with a <display-name>.
• A shared singleton: one definition, N incoming <content-link>s. The same block — and its identical children — appears everywhere it's linked; there is no per-placement override.
• A Layout block keeps its regions; its region children stay plain component.* (converting a block does not convert its children). Blocks can nest inside other blocks.
• "Convert to Content Block" rewrites an element's <type> from component.* to fragment.* and adds a <display-name> on the same content object — same content-id, no link rewiring.
• No XSD change needed: the instance's library.xsd (26.6) is byte-identical to the bundled copy and fragment.* already validates.

Changes

SDK (@salesforce/b2c-tooling-sdk)

• Add FRAGMENT to LibraryNodeType; classify fragment.* content as FRAGMENT.
• Parse and expose LibraryNode.displayName.
• Add Library.getContentBlocks() — a deduplicated catalog of a library's blocks, including unlinked ones (scans the full content set, not just the linked tree).
• Render blocks distinctly in getTreeString ((CONTENT BLOCK: <typeId>), magenta); promote and count fragments in exportContent (new fragmentCount).

CLI (@salesforce/b2c-cli)

• content export/content list render blocks distinctly and count them separately.
• content list --type fragment lists the deduplicated catalog.

VS Code extension (b2c-vs-extension)

• Per-library Content Blocks group is the single source of truth for a block (full child subtree). Because blocks are shared singletons, every page/component that links a block shows a non-expanding reference (↗) that reveals the canonical block in the group — so a block is only ever edited in one place.
• Right-click Convert to Content Block on a component (rewrites <type> + adds display-name, re-imports the element).
• Extract a reusable importLibraryXML helper; remove a leftover debug temp-file write in content-fs-provider.ts.

Docs/skills — updated the CLI content reference, the VS Code extension overview, and the b2c-content skill.

Notes

• The extension tree/convert UI builds and typechecks and reuses the proven siteArchiveImport path, but could not be exercised in the Extension Dev Host headlessly — confirm the tree rendering and convert round-trip interactively before release.
• convertToBlock is a prototype write-action; it mirrors the platform's observed mutation.