Describe dragging a selection and joining groups (#241)

- Scene editing: the group dialog offers existing groups, the folder
  stands in the icon column with the count left of it, and dragging a
  selected row moves the whole selection.
- Development: check sum 1447.

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

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                 # 1443 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 1447 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

@@ -152,8 +152,10 @@ play mode entities carry the same `parent` link: moving a parent entity moves it
 **Groups in the hierarchy (#241).** A group is a folder for top-level objects — 100 crates under
 one foldable row, say. It is editor looks only: parents, positions and behaviors stay exactly as
 they are, and the game never sees it. Select one or more top-level objects, right-click →
-**Group…**, name it; the row shows a folder, the name and the number of members, the members
+**Group…**, name it; the row shows the name, the number of members and a folder in the icon column
-indented below it and their own children one step further in. Only top-level objects go in: the
+of the objects, the members indented below it and their own children one step further in. The
+dialog's **Group** dropdown also lists the existing groups: pick one and **Add** puts the objects
+into it instead of making a new group. Only top-level objects go in: the
 entry is greyed out as soon as a child is selected, and an object put under a parent leaves its
 group. There are no groups inside groups.
 
@@ -170,7 +172,9 @@ group. There are no groups inside groups.
   before.
 - **Dragging** an object onto the group row puts it in behind the last member; dropping it into
   the gap between two members puts it there, dropping it beside a row outside the group takes it
-  out. The group row itself drags as one block between top-level rows.
+  out. The group row itself drags as one block between top-level rows. Dragging a row that is
+  part of a multi-selection moves all selected objects, side by side in their listed order and as
+  one undo step; a selected child of a selected object travels with its parent.
 - **The group row's menu** has *Select members*, *Rename…* and *Ungroup* (the members stay).
   The object menu adds *Remove from group*. Grouping, renaming, ungrouping and moving in or out
   are one undo step each.