feat(web): lazy per-script font loading for international CAD text

The web build has no system-font access, so CAD text in non-Latin scripts
(Cyrillic, Greek, CJK, …) rendered as nothing — build_fallback was a hard
None on wasm (#141). Instead, ship per-script Noto subsets under web/fonts
(one alphabet per file, SIL OFL 1.1) and fetch them lazily over HTTP the
first time a drawing uses that script, then outline glyphs with ttf-parser.

- web_font: Script enum + script_of (char → script font), a per-script
  store with lazy fetch, and a pending queue drained by the app loop.
- CJK is split by language (chinese/japanese/korean). Han ideographs share
  code points but differ by language, so the shared block is routed by the
  document's $DWGCODEPAGE (932→JP, 949→KR, GB/936→CN); kana is always
  Japanese, Hangul always Korean. Re-tessellates when the language changes.
- build_fallback (wasm) outlines from the fetched subset; clear_fallback_cache
  lets glyphs that missed while a font was in flight reappear on load.
- PollWebFonts subscription (web only) + WebFontLoaded message drive fetches.
- Trunk copy-dir serves web/fonts; nothing is bundled into the native binary
  (desktop keeps using system fonts).

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/mod.rs

@@ -794,6 +794,11 @@ pub enum DsField {
 #[derive(Debug, Clone)]
 pub enum Message {
     Tick(Instant),
+    /// Web: periodic check for per-script fonts a drawing needs but hasn't
+    /// fetched yet (#141). Native: never emitted.
+    PollWebFonts,
+    /// Web: a per-script font finished fetching — `Ok(bytes)` or `Err(reason)`.
+    WebFontLoaded(crate::scene::text::web_font::Script, Result<Vec<u8>, String>),
     OpenFile,
     /// File picker returned. `Some((path, size_in_bytes))` → start loading;
     /// `None` → user cancelled the dialog (no overlay shown).