Improve rendering pipeline to be much more selective when to render

[schemen]

• Widget reuse: Added MetadataWidget.eq() so CodeMirror reuses DOM when values/styles don’t change, avoiding re‑render per keystroke (editor-metadata.ts).
• Selection behavior: Only reveal raw syntax when the cursor is inside a marker, not for selection ranges; reduces rebuilds during selection (editor-metadata.ts).
• Fast‑path for plain values: Skip MarkdownRenderer when the value has no markdown hints; just set textContent (markdown-render.ts).
• Range scanning skip: Avoid regex scanning a range if it doesn’t contain the opener ({{ / [%) (editor-metadata.ts).
• Rebuild gating: On edits, map decorations and rebuild only if the change touches a marker or frontmatter, or if the cursor enters/leaves a marker (editor-metadata.ts).
• Per‑line marker cache: Cache marker positions per line and only re‑scan changed lines instead of full visible ranges (editor-metadata.ts).
• Frontmatter lookup caching: Added a lazy resolver with per‑key memoization and case‑insensitive key‑map caching; no cache is created unless a marker is actually resolved (metadata-utils.ts, editor-metadata.ts, metadata-renderer.ts).
• Empty‑value click fix: Empty widgets now pass events through + have a tiny hit‑box, preventing cursor misplacement without extra rendering (editor-metadata.ts, styles.css).