Build: resolve a relative CRAFT_FONTS_DIR against the workspace root

A build script's cwd is its package root, so CRAFT_FONTS_DIR=../craft-fonts —
the spelling the README and craft-fonts' integration docs both use, written
relative to the workspace root — resolved to crates/craft-fonts and failed.

The failure is silent from the user's side: the app runs, the language menu
lists every language, and selecting Chinese or Japanese paints boxes. Warn
when no font ends up embedded, not only when the variable is set but wrong.

crates/engine/build.rs

@@ -7,16 +7,20 @@
 //! `.wasm` within the web size budget (Cloudflare serves at most 25 MiB per file); the export
 //! watermark falls back to it there. Native builds embed every font.
 use std::fmt::Write as _;
-use std::path::PathBuf;
+use std::path::{Path, PathBuf};
 
 fn main() {
     println!("cargo::rerun-if-changed=build.rs");
     println!("cargo::rerun-if-env-changed=CRAFT_FONTS_DIR");
     println!("cargo::rerun-if-env-changed=CRAFT_FONTS_REQUIRED");
     let mut src = String::from("pub static CRAFT_FONTS: &[CraftFont] = &[\n");
-    if let Some(dir) = std::env::var_os("CRAFT_FONTS_DIR").map(PathBuf::from) {
+    let mut embedded = 0;
+    if let Some(dir) = std::env::var_os("CRAFT_FONTS_DIR").map(PathBuf::from).map(resolve) {
         match craft_fonts(&dir) {
-            Ok(entries) => src.push_str(&entries),
+            Ok(entries) => {
+                embedded = entries.lines().count();
+                src.push_str(&entries);
+            }
             Err(e) if std::env::var_os("CRAFT_FONTS_REQUIRED").is_some() => {
                 println!("cargo::error=CRAFT_FONTS_DIR={}: {e}", dir.display());
             }
@@ -24,14 +28,37 @@ fn main() {
         }
     }
     src.push_str("];\n");
+    // Without them the app still runs and every language still selectable, so Chinese and Japanese
+    // just paint boxes — the one failure a user cannot diagnose from what they see.
+    if embedded == 0 {
+        println!(
+            "cargo::warning=no craft-fonts embedded: Chinese and Japanese text will render as empty boxes. \
+             Clone https://github.com/storytold/craft-fonts next to this repo and rebuild with \
+             CRAFT_FONTS_DIR=../craft-fonts"
+        );
+    }
     let out = PathBuf::from(std::env::var_os("OUT_DIR").unwrap_or_default()).join("craft_fonts.rs");
     if let Err(e) = std::fs::write(&out, src) {
         println!("cargo::error=writing {}: {e}", out.display());
     }
 }
 
+/// Resolve a relative `CRAFT_FONTS_DIR` against both roots a caller might mean: a build script's
+/// cwd is the *package* root (`crates/engine`), while the `../craft-fonts` the README and
+/// craft-fonts' docs use is written relative to the *workspace* root and would otherwise resolve
+/// to `crates/craft-fonts`.
+fn resolve(dir: PathBuf) -> PathBuf {
+    if dir.is_absolute() {
+        return dir;
+    }
+    let package = PathBuf::from(std::env::var_os("CARGO_MANIFEST_DIR").unwrap_or_default());
+    let workspace = package.parent().and_then(Path::parent).unwrap_or(package.as_path());
+    let candidates = [workspace.join(&dir), package.join(&dir)];
+    candidates.iter().find(|candidate| candidate.is_dir()).cloned().unwrap_or_else(|| candidates[0].clone())
+}
+
 /// One `CraftFont { .. }` initialiser per manifest line.
-fn craft_fonts(dir: &std::path::Path) -> Result<String, String> {
+fn craft_fonts(dir: &Path) -> Result<String, String> {
     let manifest = dir.join("fonts/manifest.txt");
     println!("cargo::rerun-if-changed={}", manifest.display());
     let text = std::fs::read_to_string(&manifest).map_err(|e| format!("{}: {e}", manifest.display()))?;