Preview source-owned dependency edges in the projection dry run (STATE-WP-0094-T05)

diff_against_hub accepts hub_edges/edge_targets and reports edges to create,
remove, retain, unresolved and external, using the same pure planner that
receiving now calls (plan_source_edges + classify_source_edges). The classifier
works on a row list, fixing a collapse where an external duplicate of an owned
edge key could hide the owned row from removal. 7 new pure tests.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 692292@bnt-lap001
Assistant-Session: 89c7d8e8-e1b3-47eb-9411-a0ab227fca2f

docs/forge-dependency-edge-receiving.md

@@ -67,6 +67,24 @@ ownership contract first.
 Outcome schema additions (`state-hub.projection-reset.v1`): `created_edges`,
 `removed_edges`, `unresolved_edges`, `external_edges` lists and their counts.
 
+## Dry-run preview
+
+`diff_against_hub(..., hub_edges=[...], edge_targets={...})` previews edges
+with the same pure planner receiving uses (`plan_source_edges` +
+`classify_source_edges`). `hub_edges` are the rows whose from-workplan belongs
+to the repository (with `source_owner`); `edge_targets` maps a target record id
+to its fleet-wide non-retired candidate ids. A target the hub lacks but this
+projection declares resolves to the id the receipt would create. The diff gains
+`edges` (`create`, `remove`, `retain`, `unresolved`, `external`) and
+`edge_counts`; only `create`/`remove` make it non-clean. Without `hub_edges`
+the output is unchanged (no `edges` key).
+
+Duplicate keys: several rows may share one edge key (an owned row plus an
+API-authored duplicate). The classifier works on the row list, so an external
+duplicate can neither hide an owned row from removal nor turn a satisfied
+declaration into an external report (fixed 2026-10-04; the first
+implementation keyed rows by edge key and let the last row win).
+
 ## Coexistence with C-20
 
 C-20 keys existence on (from task, target, relationship) regardless of owner,
@@ -78,7 +96,6 @@ explicit adoption decision.
 
 - Pre-existing C-20/API edges are never adopted, so a removed declaration does
   not remove an edge C-20 created earlier; adoption needs its own decision.
-- `diff_against_hub` (dry run) does not yet preview edge changes.
 - Ambiguity by slug for workplans is mostly prevented by the global slug index;
   task `record_id` ambiguity across workplans is the realistic case.
 - Not deployed; no evidence of scheduled freshness. T05 stays in progress.