Open a sprite's texture or shader from the menus and inspector (#221)

A sprite's texture and shader were only reachable by hunting for them
in the asset panel. Every place that shows such a reference now leads
straight to it.

- Object menu (hierarchy and scene view): "Open texture" and "Open
  shader" sit right below "Open asset" when the sprite has one. The
  object's own values count, so an overridden shader opens the
  override.
- Asset context menu: the same entries at the top.
- Inspector: every asset field with a value shows its label as a blue,
  underlined link. One click selects the asset, a double click opens
  it. An override stays gold and gets the underline too.
- The first click rebuilds the inspector, so the second one lands on a
  different row. The inspector catches that second click before any
  row sees it.
- New pure rule `references.ts`: the asset behind a reference field. A
  frame leads to its sheet. Kinds without a tab (palette, font, sound)
  get no menu entry.
- `followsPress` in `clicks.ts` keeps the double-click window and
  tolerance in one place.
- `activateAsset` in `actions.ts`: opening means the same everywhere
  as a double click in the asset panel, so a scene also becomes the
  active one. The asset panel uses it too.

Tests:
- ./check.sh: 150 files, 2226 tests, CHECK EXIT 0 (before 148/2214).
- 12 new unit tests: 6 for the reference rule, 4 for the link in
  createFieldRow, 2 for followsPress.
- 5 new e2e checks in 28l-ii: both menus, the link's look, click
  selects, double click opens.
- e2e 1-16,26,28,35,39,40 on the final build: ALL PASSED, 732 checks.
- Counter-probes: no "Open ..." entries, no underline and a second
  click never recognised each fail exactly at their checks.
- Check total 1341 (CI counted 1336 at fd6af84), computed.

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

src/core/properties/references.ts

@@ -0,0 +1,37 @@
+import { type PropertyBag, type PropertyValue, textOf } from '../model/Properties';
+import { findPixelFrame } from '../model/pixelart';
+import { findAsset, type Project } from '../model/Project';
+import { findFrame } from '../model/texture';
+import { isKindWith } from './kinds';
+import type { PropertyDef } from './PropertySchema';
+
+/**
+ * The asset a reference field points at, null when it names nothing or no longer resolves. A frame
+ * names the texture or pixel art it was cut from – that is the asset one can open (#221).
+ */
+export function referencedAssetId(project: Project, def: PropertyDef, value: PropertyValue): string | null {
+  if (!isKindWith('reference', def.kind)) return null;
+  const id = textOf(value);
+  if (id === '') return null;
+  if (findAsset(project, id)) return id;
+  if (def.kind !== 'texture') return null;
+  return findFrame(project, id)?.asset.id ?? findPixelFrame(project, id)?.asset.id ?? null;
+}
+
+export interface Reference {
+  label: string;
+  assetId: string;
+}
+
+/** Every reference of a bag that resolves, in schema order – the "Open …" entries of a menu. */
+export function referencesOf(
+  project: Project,
+  schema: readonly PropertyDef[],
+  values: PropertyBag,
+): Reference[] {
+  return schema.flatMap((def) => {
+    const value = values[def.key];
+    const assetId = value === undefined ? null : referencedAssetId(project, def, value);
+    return assetId ? [{ label: def.label, assetId }] : [];
+  });
+}