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/view.rs

@@ -1630,11 +1630,20 @@ impl OpenCADStudio {
         } else {
             Subscription::none()
         };
+        // Web: poll for per-script fonts that a drawing's text needs but hasn't
+        // fetched yet. Cheap — `PollWebFonts` is a no-op when nothing is
+        // pending. Native has system fonts, so no polling. (#141)
+        #[cfg(target_arch = "wasm32")]
+        let web_fonts =
+            iced::time::every(std::time::Duration::from_millis(300)).map(|_| Message::PollWebFonts);
+        #[cfg(not(target_arch = "wasm32"))]
+        let web_fonts = Subscription::none();
         iced::Subscription::batch([
             frames,
             history_tick,
             grip_dwell,
             caret_blink,
+            web_fonts,
             event::listen_with(|ev, status, win_id| {
                 use iced::event::Status;
                 match ev {