Fonts: a new asset type, picked from the server or dropped in, worn by text objects (#74)

A font asset holds a file and nothing else, the twin of a texture: the server keeps it
content-addressed under data/fonts/{sha256}.{ext}, with the format read from the file's own
four-byte signature (finfo reports fonts inconsistently across libmagic versions). A text
wears one through its ordinary Font property, so overrides, reset, undo and the Properties
node come for free; no font means the editor's own typeface.

The picker lists everything the server holds and renders each row in the face it offers.
The family name is gm-font-<hash16>, never the name inside the file, so two "Regular" cannot
collide. WorldViewport.syncText is the single place that draws it, which covers scene, game
preview and play at once.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/store/serialize.ts

@@ -15,6 +15,7 @@ import {
 import { isShaderNodeDef } from '../shader/nodes';
 import type { Asset, TextureFrame } from '../model/Asset';
 import { isAssetType } from '../model/AssetTypes';
+import { isFontFile } from '../model/font';
 import { emptyEdit, type InstanceEdit, isEmptyEdit } from '../model/instance';
 import {
   type FieldValue,
@@ -116,6 +117,9 @@ export function deserializeProject(raw: unknown, fallbackId = ''): Project {
       assets.push({ id, type, name, props, graph: shaderGraphs.get(id) ?? createEmptyGraph() });
     } else if (type === 'texture') {
       assets.push({ id, type, name, props, ...normalizeTexture(a, takenIds) });
+    } else if (type === 'font') {
+      const file = typeof a.file === 'string' && isFontFile(a.file) ? a.file : '';
+      assets.push({ id, type, name, props, file });
     } else if (type === 'animation') {
       // Kept even when they dangle, like a sprite's `texture`: a frame reads as no picture.
       const frames = Array.isArray(a.frames) ? a.frames : [];