md-tree integration

Merlon Docs uses @kayvan/markdown-tree-parser for split only (extract-all, explode).

Upstream assemble is not used, because it does not support:

• Per-section heading demotion (shard # → compiled ##)
• The preamble shard handling that Project layout describes

When to fork

Vendor a fork into packages/markdown-tree when:

1. Merlon Docs moves compile onto md-tree’s assemble, which then needs heading-transform hooks
2. explode must preserve the preamble without synthetic H2 promotion
3. Upstream is unresponsive or breaking

Part of Design constraints.