docs: explain upward propagation with part_integrity="cascade"

Describe how cascade propagates restrictions upward from part to
master, then back downstream to all sibling parts, deleting the
entire compositional unit. Updated in both diagram.md and
master-part.md.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/reference/specs/diagram.md

@@ -151,7 +151,9 @@ Prepare a cascading delete. Starting from a restricted table expression, propaga
 |-------|----------|
 | `"enforce"` | Error if parts would be deleted before masters |
 | `"ignore"` | Allow deleting parts without masters |
-| `"cascade"` | Also delete masters when parts are deleted |
+| `"cascade"` | Propagate restriction upward from part to master, then re-propagate downstream to all sibling parts |
+
+With `"cascade"`, the restriction flows **upward** from a part table to its master: the restricted part rows identify which master rows are affected, those masters receive a restriction, and that restriction propagates back downstream through the normal cascade — deleting the entire compositional unit (master + all parts), not just the originally matched part rows.
 
 ```python
 # Build a cascade from a restricted table

src/reference/specs/master-part.md

@@ -216,10 +216,9 @@ Session.Trial.delete()
 (Session.Trial & condition).delete(part_integrity="cascade")
 ```
 
-**Behavior:**
-- Identifies affected masters
-- Deletes masters (which cascades to ALL their parts)
-- Maintains compositional integrity
+**Behavior:** The restriction propagates **upward** from the part to its master. Specifically, the restricted part rows identify which master rows are affected, and those masters receive a restriction. The master restriction then propagates back **downstream** through the normal cascade, reaching all sibling parts. The result is that the entire compositional unit — master plus all its parts — is deleted, not just the originally restricted part rows.
+
+This upward propagation may trigger further rounds: if the master itself is a part of a higher-level master, the restriction continues upward. The cascade engine iterates until no new restrictions are produced.
 
 ### 4.6 Behavior Matrix