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 <noreply@anthropic.com>

Cargo.toml

@@ -91,8 +91,12 @@ serde_json = "1"
 [target.'cfg(target_arch = "wasm32")'.dependencies]
 console_error_panic_hook = "0.1"
 wasm-bindgen = "0.2"
+# fetch() returns a JS Promise; bridge it to a Rust future for the per-script
+# web font loader (#141).
+wasm-bindgen-futures = "0.4"
 js-sys = "0.3"
-# window.open (external URLs) + Blob/anchor file downloads (Save).
+# window.open (external URLs) + Blob/anchor file downloads (Save) + fetch()
+# (Response) for the lazy per-script web font loader (#141).
 web-sys = { version = "0.3", features = [
     "Window",
     "Navigator",
@@ -101,6 +105,7 @@ web-sys = { version = "0.3", features = [
     "HtmlAnchorElement",
     "Blob",
     "Url",
+    "Response",
 ] }
 # Browsers without WebGPU (e.g. Firefox by default) need wgpu's WebGL2 backend;
 # `webgl` enables it. `fira-sans` embeds the default UI font — the web has no