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).

src/app/update.rs

@@ -364,6 +364,42 @@ impl OpenCADStudio {
 
     fn update_inner(&mut self, msg: Message) -> Task<Message> {
         match msg {
+            // Web: a drawing referenced a script whose Noto subset isn't loaded
+            // yet (recorded during text tessellation). Kick off one fetch per
+            // pending script; the result comes back as `WebFontLoaded`. (#141)
+            Message::PollWebFonts => {
+                let pending = crate::scene::text::web_font::take_pending();
+                if pending.is_empty() {
+                    return Task::none();
+                }
+                Task::batch(pending.into_iter().map(|script| {
+                    Task::perform(crate::scene::text::web_font::fetch(script), move |res| {
+                        Message::WebFontLoaded(script, res)
+                    })
+                }))
+            }
+
+            // Web: a per-script font arrived. Store it, drop the stale fallback
+            // glyph cache (entries that resolved to nothing while it loaded),
+            // and re-tessellate so the text appears. (#141)
+            Message::WebFontLoaded(script, res) => {
+                match res {
+                    Ok(bytes) => {
+                        crate::scene::text::web_font::insert(script, Some(bytes));
+                        crate::scene::text::ttf_glyph::clear_fallback_cache();
+                        for tab in self.tabs.iter_mut() {
+                            tab.scene.bump_geometry();
+                        }
+                    }
+                    Err(e) => {
+                        crate::scene::text::web_font::insert(script, None);
+                        self.command_line
+                            .push_error(&format!("Font load failed ({script:?}): {e}"));
+                    }
+                }
+                Task::none()
+            }
+
             Message::Tick(t) => {
                 let i = self.active_tab;
                 self.tabs[i].scene.update(t - self.start);
@@ -500,6 +536,15 @@ impl OpenCADStudio {
 
                 self.tabs[i].current_path = Some(path.clone());
                 self.tabs[i].scene.document = doc;
+                // Route shared CJK ideographs to the language matching this
+                // drawing's code page (web per-language font split). Drop the
+                // glyph cache if it changed so Han re-resolves to the new
+                // language's font; geometry is (re)built below regardless. (#141)
+                if crate::scene::text::web_font::set_cjk_lang_from_codepage(
+                    &self.tabs[i].scene.document.header.code_page,
+                ) {
+                    crate::scene::text::ttf_glyph::clear_fallback_cache();
+                }
                 // Current model-space annotation scale comes from the drawing's
                 // CANNOSCALEVALUE (paper/drawing factor); the multiplier we use
                 // for text/dim sizing is its inverse (1:50 -> 0.02 -> 50.0).
@@ -1298,6 +1343,14 @@ impl OpenCADStudio {
                     self.sync_ribbon_from_selection();
                     // Grid/snap follow the newly active drawing's viewport.
                     self.adopt_view_display(idx);
+                    // Shared CJK ideographs follow the newly active drawing's
+                    // language; re-tessellate if it differs from the last. (#141)
+                    if crate::scene::text::web_font::set_cjk_lang_from_codepage(
+                        &self.tabs[idx].scene.document.header.code_page,
+                    ) {
+                        crate::scene::text::ttf_glyph::clear_fallback_cache();
+                        self.tabs[idx].scene.bump_geometry();
+                    }
                 }
                 Task::none()
             }

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 {