Frontmatter: leading YAML run is one pinned block; example renders a Properties card

#10 in Riparion/riparion-editor — merged 2026-06-05

Summary

Ergonomic YAML frontmatter handling, end to end:

Core (src/lib.rs)

• split_blocks consumes a leading ---…--- run (byte offset 0 only) as the outermost rule — before fences, is_atomic, and blank-line breaks — so its interior can never be split. The blank separator after the closing delimiter attaches to the block, same as every other block's trailing blank.
• New Block::frontmatter flag and public frontmatter_len() helper.
• rejoin_normalized preserves the YAML verbatim, so normalize-on-blur can't restructure it.
• Pinned in arrange mode: the block renders above the DragDropArea with no ⠿ handle and no drop slot, plus a defensive guard in on_drop — reordering can't move it or land anything above it.
• Unclosed --- stays an ordinary thematic break (doesn't swallow the doc); ... closes per YAML; CRLF-safe; byte-for-byte round-trip invariant holds.

Markdown example

• Obsidian-style Properties card: scalar key: value rows edit inline (click value; Enter/blur commits, Esc cancels), per-row remove, + Add property.
• Non-scalar values (tags: [a, b], nested blocks, comments) render read-only; the {} YAML button bubbles to the block's normal activate — the raw textarea is the escape hatch, for free.
• Commits are line-targeted YAML rewrites, so comments, ordering, and the body survive untouched (no serializer round-trip).
• File ▸ Add properties prepends starter frontmatter when missing; SEED shows the card off.

Test plan

• 14 new unit tests: one-block splitting, hostile interiors (embeds/fence chars inside YAML), offset-0-only detection, leading-blank/unclosed/empty/CRLF/...-closed runs, frontmatter-only docs, normalization (38 total pass)
• cargo clippy --all-targets --features web clean; cargo fmt --check clean
• cargo check --example markdown --features web --target wasm32-unknown-unknown
• Manual: dx serve --example markdown --features web — card pinned at top, inline edits persist, drag can't displace it, Save round-trips

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