app: reverse-DNS app id and the Windows executable icon

The window's Wayland app_id / X11 class is io.github.jyviko.fieldcraft, the id the release
packages use for the .desktop entry and the macOS bundle (#72). Windows builds embed
assets/app-icon/icon.ico in fieldcraft.exe through embed-resource (MIT, a build dependency only).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NmAttHhUZTokgPeEJia8sm

apps/fieldcraft/build.rs

@@ -0,0 +1,20 @@
+//! Windows builds embed the application icon (assets/app-icon/icon.ico) in `fieldcraft.exe`, so
+//! Explorer, the taskbar and shortcuts show it. Other targets need nothing here.
+#![forbid(unsafe_code)]
+
+use std::path::PathBuf;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let icon = PathBuf::from(std::env::var("CARGO_MANIFEST_DIR")?).join("../../assets/app-icon/icon.ico");
+    println!("cargo:rerun-if-changed=build.rs");
+    println!("cargo:rerun-if-changed={}", icon.display());
+    if std::env::var("CARGO_CFG_TARGET_OS")? != "windows" {
+        return Ok(());
+    }
+    let out = PathBuf::from(std::env::var("OUT_DIR")?);
+    let rc = out.join("fieldcraft.rc");
+    std::fs::write(&rc, format!("1 ICON \"{}\"\n", icon.display().to_string().replace('\\', "/")))?;
+    // Missing resource tools are a build error, rather than silently shipping a generic icon.
+    embed_resource::compile_for(&rc, ["fieldcraft"], embed_resource::NONE).manifest_required()?;
+    Ok(())
+}