fix: squash infinite frontmatter duplication loop and add YAML guardrails

[kavinsood]

A user shared a duplication bug caused by frontmatter. The failure mode:

• The Catalyst (External Edits): A concurrent offline edit (likely from a third-party plugin like TaskNotes or Obsidian's property UI) introduced a slightly malformed YAML state. Because YAOS currently treats the entire file as a single raw Y.Text string, the CRDT blindly merged the bytes, creating syntactically broken YAML.
• The Amplifier (The P0 Bug): When YAOS detected a local-only divergence, our recovery path applied the disk content to the CRDT... but then immediately called editorBindings.heal(), which forcefully slapped the stale editor content back into the CRDT. This created a feedback loop that exponentially duplicated the YAML properties.

The fix:

This PR stops the bleeding and builds a shield around the CRDT.

Patched the Amplifier: In main.ts, I swapped the content-writing heal() call for a non-writing repair() call during bound-file recovery. The infinite append loop is dead.

Frontmatter Containment Guard: Added a lightweight, structure-aware classifier (frontmatterGuard.ts). Before frontmatter crosses the Disk ↔ CRDT bridge, YAOS now intercepts duplicate top-level keys, broken --- fences, and unnatural growth bursts.

Fail-Open Quarantine: If external YAML is malformed, YAOS will quarantine the update (logging a trace) so the CRDT doesn't get corrupted, while the main body text continues syncing seamlessly.