Describe Sort with parent for children (#232)

A child's Z used to count against the whole scene. The new switch keeps
it local to the parent, and the pages need to say when and how.

- Scene editing: new paragraph "Sort with parent". It covers behind and
  in front of the parent, chains, the defaults for new and saved
  objects and the parents where it has no effect.
- Animations: the Z row points to Sort with parent.
- Development: check total 1398.

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

Animations.md

@@ -15,7 +15,9 @@ A placed root or object offers its position as the first rows: **X**, **Y** and
 draw order (higher is on top), and the order is sorted again on every render, so a keyed *Z* moves
 the object behind or in front of others while the animation plays — a weapon that swings up can
 pass behind its character. Blended values between two keys count as they are: at *Z* 4.5 the
-object is above everything on 4 and below everything on 5.
+object is above everything on 4 and below everything on 5. A child with *Sort with parent* on
+moves only against its parent and siblings, not against the whole scene (see
+[Scene editing](Scene-editing), #232).
 
 Besides the schema's own properties, a row can key what an object **declares** (#104): a
 **behavior's variables** — pick the behavior (it sits below its owner in the object list) and its

Development.md

@@ -15,7 +15,7 @@ npm run build        # writes dist/ and regenerates index.html
 php tools/make-user.php <name> <password> [admin|user]        # an account; every tool signs in (see Accounts)
 php tests/php/accounts.test.php                               # the API's account rules, no server needed (CI's gate job)
 php tests/php/snapshots.test.php                              # the snapshot store (#231), the same way
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1397 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1398 editor checks, screenshots in /tmp/gm-e2e-*.png
 GM_STEPS=20 node tools/e2e.mjs                                # or 1-12, or 6,20 — a subset while working on one step
 ./e2e.sh start [GM_STEPS]                                     # the same run detached, see below
 ./e2e.sh status                                               # live progress, or the last run's result

Scene-editing.md

@@ -66,6 +66,19 @@ of a tilemap are sorted in one by one, each at the map's `z` plus its own (see [
 On the `z` of a layer sorted by Y, Y decides instead: whatever stands higher up is drawn first
 (see [Tilemaps](Tilemaps#sorting-by-y), #228).
 
+**Sort with parent.** A child's `z` adds up with its parent's, and on its own the child is sorted
+against the whole scene: at `z` -1 under a figure on 0 it would fall behind *everything* on 0.
+Sprites, texts and particle sources therefore have the switch **Sort with parent**, right under
+*Z* in the inspector (#232). With it on, the child is drawn in its parent's place: against the rest
+of the scene only the parent counts, with its `z` and, on a `z` sorted by Y, its feet. The child's
+own `z` then only orders it against the parent and their siblings: below 0 behind the parent, 0 and
+above in front of it, ties in hierarchy order. It chains, so a grandchild with the switch on sorts
+with the topmost object of the chain. A new asset starts with the switch on; an asset stored before
+it had it reads as off, so older projects draw as they did. It is an ordinary property: set on the
+asset, overridden per object. A parent that is not drawn itself (container, behavior, camera,
+collider) or is a tilemap has no single place to share, so there the switch does nothing. The rule
+lives in `drawSlots` (`src/viewport/drawOrder.ts`), which the scene view, the game and picking share.
+
 **Transform.** Sprites, texts, colliders and containers carry four more rows next to their
 position: **Scale** (one factor, default 1), **Rotation** (degrees, positive turns clockwise on
 screen), **Mirror X** and **Mirror Y**. They are ordinary properties — set on the asset, overridden