Describe the size gizmo in the scene view (#223)

Scene-editing explains edges, corners and the turn/scale knob. Development has the new check total 1363.

Development.md

@@ -14,7 +14,7 @@ npm run build        # writes dist/ and regenerates index.html
 ./build.sh           # the same under `set -o pipefail`, for callers that pipe the output
 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)
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1357 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1363 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

@@ -59,6 +59,23 @@ the corners; dragging an object moves it in its parent's frame, so a child under
 by a quarter walks down the screen when dragged along its own x. `Move` and `Set position` work
 in the object's own axes as well.
 
+**Size gizmo.** A selected sprite or collider gets handles on its outline (#223), turned with it:
+- **Edges and corners** set Width and Height (a corner both). The opposite side stays where it was,
+  so the object's position moves by half the change. Sizes land on whole units, at least 1.
+  A handle reaches 7 pixels outwards but only half that into the object, so a small sprite can
+  still be grabbed and moved close to its edge.
+- A **circle collider** has edge handles only: any edge sets the radius, and the circle stays round.
+  A polygon or frame collider has no size handles, its corners are edited on the outline itself.
+- The **knob** on a stem above the top edge turns *or* scales the object. The first few pixels of
+  the drag decide which: pulled away from or towards the centre it scales, pulled sideways it
+  turns. That choice holds until the release, so one drag never does both. Shift snaps a turn to
+  15°. The turn is the angle swept round the object's centre, so it can go on past a half circle.
+- The values go into **this object's overrides**, like "Fit to sprite": a value equal to the
+  asset's own clears the override. One drag is one undo step.
+
+The rules are in `src/editor/sizeGizmo.ts`, the handles in
+`src/editor/viewport/sizeGizmoEditor.ts`.
+
 **Hierarchy (parenting).** Every scene object may have a parent in the same scene
 (`SceneObject.parentId`). Positions — including `z` — are **local to the parent**; the world
 position is the parent's transform applied to the local one, all the way up (`z` simply adds).