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>

tests/ui/clicks.test.ts

@@ -1,6 +1,6 @@
 import type { FederatedPointerEvent } from 'pixi.js';
 import { describe, expect, it } from 'vitest';
-import { pressCount } from '../../src/ui/clicks';
+import { followsPress, pressCount, pressOf } from '../../src/ui/clicks';
 
 interface Press {
   time: number;
@@ -51,3 +51,17 @@ describe('pressCount', () => {
     expect(counts('row-h', [{ time: 0 }, { time: 100 }, { time: 200 }, { time: 300 }])).toEqual([1, 2, 1, 2]);
   });
 });
+
+describe('followsPress', () => {
+  // The second press of a link's double-click lands on a rebuilt panel, so no key is compared (#221).
+  const first = pressOf(press({ time: 0 }));
+
+  it('joins a press soon after at the same spot', () => {
+    expect(followsPress(first, press({ time: 300, x: 104 }))).toBe(true);
+  });
+
+  it('does not join a late press or one elsewhere', () => {
+    expect(followsPress(first, press({ time: 500 }))).toBe(false);
+    expect(followsPress(first, press({ time: 100, y: 120 }))).toBe(false);
+  });
+});