Add incremental adapter compiler boundary

docs/MCP_CONTRACT.md

@@ -49,6 +49,7 @@ gate.
 - `docforge_propose_node_create`
 - `docforge_propose_node_update`
 - `docforge_propose_node_move`
+- `docforge_propose_relationship_update`
 - `docforge_propose_node_delete`
 - `docforge_validate_changeset`
 - `docforge_get_changeset_diff`
@@ -58,6 +59,8 @@ Proposal tools may write only below the configured changeset or isolated preview
 change canonical files or declared project output. Without `--proposal-writer`, changeset mutation
 tools return `proposal_access_disabled`. Validation, diff retrieval, and preview remain available
 for existing changesets. A preview accepts a declared view ID, not a renderer name or command.
+The relationship-update tool queues additions and removals without rewriting node content and
+rejects an empty relationship list.
 
 ## Canonical application tool