Show a placed object's struct the way its asset does

The instance inspector drew its own, flatter version of a struct: one row per
field, no bundle head. It now draws the same block as the asset side, because
there is only one implementation left - both call buildStructBlock. What the
instance does not get is everything that touches the shape: renaming a field,
'+ Field', and the per-field '×'. The shape belongs to the asset; a placed
object overrides values. The revert and override marks sit on the head rather
than on a field, since an override covers the whole bundle at once.

Underneath this, editing a single field of an unoverridden struct used to blank
its siblings: structFieldsOf filled the untouched fields from spec.read, which
is object.overrides[key] and therefore undefined until something is overridden.
structOf then handed back the empty shape. PreviewedValue carries 'current' now
- the resolved value the row actually shows - and the fields come from that.
Without it the old path stays, which is right for the asset side, whose prop
bag is dense.

README.md

@@ -120,8 +120,10 @@ shown on the node and editable in the inspector.
   has, one level down and with a value column – and a field may itself be a **list**, which is
   the one count button that opens the tab one level deeper. Adding, renaming or removing a field reaches the
   declaration, the behavior's own value **and every instance override** in one command, so no
-  stored value keeps a shape its declaration no longer promises. A placed instance sees the same
-  fields as plain value rows: it may override them, not declare them.
+  stored value keeps a shape its declaration no longer promises. A placed instance draws the very
+  same block (`buildStructBlock`), minus the renaming and the `+ Field` / `×` buttons: it may
+  override the values, not declare them. Its ↺ / ↑ sit on the head row, because an override covers
+  the whole bundle rather than a single field.
 - **Owner** = parent: a placed behavior is usually parented to the object it controls
   (inspector → Parent, or drag it onto that object in the hierarchy; dashed line in the
   viewport). The `Owner` node returns it; `Self` returns the behavior itself; `Parent` walks up
@@ -607,7 +609,7 @@ npm run check        # typecheck + lint + format:check + tests, what CI would ru
                      #   code is lost (tail's is reported), so a red run reads as green
 npm run build        # writes dist/ and regenerates index.html
 export GM_REMOTE=user@host                                    # optional: run the browser elsewhere; see below
-node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 502 editor checks, screenshots in /tmp/gm-e2e-*.png
+node tools/seed-e2e.mjs && node tools/e2e.mjs                 # 509 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