Clean up duplicates and dead code, and fix the + menu check (#85)

Refactoring round 9 covers everything since round 8 (14972d0): 62
commits with about 29 000 new lines in src, api, tools and tests.
Behaviour is unchanged. The one fix is in a check: e2e 04 counted 25
asset types, and the shader function from #303 makes 26, so the run
of a526e1b failed there.

Duplicate code (jscpd over src, 50 tokens, without the two licence
texts: 35 clones / 289 lines before, 29 / 232 after):

- ShaderPicture (new, viewport/shaderPicture.ts) is the picture a
  camera's shader and a particle field's shader are laid over. It
  keeps the render texture while its size holds and frees the
  replaced one once the shader holds the new one. CameraShade and
  ParticleField.shadeField drew it the same way, line for line.
- CameraShade builds its mesh on the first draw. Before, every redraw
  of a view without a camera shader built a mesh and threw it away.
- emptyHost in inspector/host.ts builds the empty host the Inspector,
  the Variables tab and the Tilemap panel start from.
- paddedQuad(null) is the plain unit quad. symbols.ts and
  shaderPasses.ts each wrote it out by hand.
- In graph/nested.ts, calls of shader functions and On behavior
  nodes are followed by one walk, `reaches`, over ids `namedBy` a
  node type.

Dead code:

- ShaderEditor.colorFieldOf and the getter CurveCanvas.snapping had
  no reader.
- 22 values used only in their own file lose their `export`. Types
  and the node catalogues of shaders and particles stay exported.
- Kept on purpose: PhysicsSystem.holdsJoint is read by tests only,
  but it is their one view of whether a joint holds.
- A pass with stricter lint rules over every file changed since
  round 8 found no bug. PixelArtEditor.started is now readonly.

Comments and docs:

- Ten comments told history ("since #190", "used to", "any more").
  They now describe what is. The bundle is md5-identical before and
  after, and the four e2e files minify to the same bytes.
- README: joint and shader function in the asset list, joints on the
  Physics row, what the Shaders page covers, two CI runners.
- e2e 04 keeps the number of asset types once, as ASSET_TYPE_COUNT.
- screenshot.png in the README is a new picture of the editor
  (3840x2160 instead of 1825x1140), taken by the user.

Tests:
- Gate on melektaus-atx: 229 files, 3514 tests, CHECK EXIT 0
  (+3 for emptyHost, +1 for the unit quad).
- e2e 1-4, 31, 63 on melektaus-atx: ALL PASSED, 195 checks. That is
  the + menu of 04, the field shader of 31 and the camera shader of
  63, both through ShaderPicture.
- No new e2e check, so the sum stays 1728.
- Counter-checks, each put back and compared with cmp: reaches that
  never goes on fails 1 test, listensTo reading the wrong parameter
  fails 1, emptyHost dropping what a panel hands in fails 1, folding
  without a relayout fails 1.

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

tools/e2e/04-assets.mjs

@@ -1,6 +1,9 @@
 import { colourDistance } from '../lib/harness.mjs';
 import { inView } from './lib/scaffold.mjs';
 
+/** One per entry of `ASSET_TYPES` – a new asset type counts here too. */
+const ASSET_TYPE_COUNT = 26;
+
 /** The asset panel: placing by dragging into the view, creating through "+", and resetting an asset value. */
 export async function assets(t) {
   const {
@@ -97,9 +100,9 @@ export async function assets(t) {
   await redo();
   check('redo puts it back', (await objects()).length === 8);
 
-  // 4c. Create a new asset through the + menu. Since #43 the menu is two alphabetical groups –
-  //     what can be placed in a scene, a line, then what only lives in the library. With the Joint
-  //     (#154) its 25 types no longer fit under the button on a canvas 800 high: there it scrolls.
+  // 4c. Create a new asset through the + menu. The menu is two alphabetical groups (#43) – what
+  //     can be placed in a scene, a line, then what only lives in the library. Its types do not
+  //     all fit under the button on a canvas 800 high: there it scrolls.
   await clickText('+');
   const rows = await json('__gm.menuRows()');
   const cut = rows.indexOf(null);
@@ -112,7 +115,7 @@ export async function assets(t) {
   );
   check(
     'every asset type is offered once, each group sorted by label',
-    placeable.length + library.length === 25 &&
+    placeable.length + library.length === ASSET_TYPE_COUNT &&
       String(placeable) === String([...placeable].sort()) &&
       String(library) === String([...library].sort()),
     JSON.stringify({ placeable, library }),
@@ -486,7 +489,7 @@ export async function assets(t) {
 async function addMenuOnRightClick(t) {
   const { check, json, shot, click, clickText, press, undo, showTab } = t;
   const addRows = (rows) =>
-    Array.isArray(rows) && rows.filter(Boolean).length === 25 && rows.includes('Sprite');
+    Array.isArray(rows) && rows.filter(Boolean).length === ASSET_TYPE_COUNT && rows.includes('Sprite');
   const heading = await json("__gm.assetRowShown('group:scene')");
   const sceneFolded = async () =>
     !(await json('__gm.assetGroups()')).find((g) => g.type === 'scene').expanded;