feat: add ahfail-display standalone binary

Implements the ahfail-display binary crate: GTK popup window that spawns
the Nedry sprite and plays the audio clip, with SIGTERM handling, 15-minute
failsafe, deadzone CLI parsing, volume save/restore, and update check.
Adds a build.rs that compiles GResources via glib-compile-resources so the
binary can be built with plain `cargo build` outside of Meson.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

crates/ahfail-display/build.rs

@@ -0,0 +1,51 @@
+use std::path::PathBuf;
+use std::process::Command;
+
+fn main() {
+    let manifest_dir = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    // Workspace root is two levels up from crates/ahfail-display/
+    let workspace_root = manifest_dir.join("../..").canonicalize().unwrap();
+    let assets_dir = workspace_root.join("assets");
+    let gresource_xml = assets_dir.join("ahfail.gresource.xml");
+
+    let out_dir = PathBuf::from(std::env::var("OUT_DIR").unwrap());
+    let c_src = out_dir.join("ahfail-resources.c");
+
+    // Re-run if any asset changes
+    println!("cargo:rerun-if-changed={}", gresource_xml.display());
+    for entry in std::fs::read_dir(&assets_dir).unwrap() {
+        let entry = entry.unwrap();
+        println!("cargo:rerun-if-changed={}", entry.path().display());
+    }
+
+    let status = Command::new("glib-compile-resources")
+        .args([
+            "--generate-source",
+            "--target",
+            c_src.to_str().unwrap(),
+            "--sourcedir",
+            assets_dir.to_str().unwrap(),
+            gresource_xml.to_str().unwrap(),
+        ])
+        .status()
+        .expect("glib-compile-resources not found — install libglib2.0-dev or equivalent");
+
+    assert!(status.success(), "glib-compile-resources failed");
+
+    // Use pkg-config to get include flags for gio-2.0
+    let gio_cflags = Command::new("pkg-config")
+        .args(["--cflags", "gio-2.0"])
+        .output()
+        .expect("pkg-config not found");
+    let gio_cflags_str = String::from_utf8(gio_cflags.stdout).unwrap();
+
+    let mut build = cc::Build::new();
+    build.file(&c_src).flag_if_supported("-w"); // suppress warnings in generated code
+    for flag in gio_cflags_str.split_whitespace() {
+        build.flag(flag);
+    }
+    build.compile("ahfail_resources");
+
+    // Link against gio-2.0 for GResource support
+    println!("cargo:rustc-link-lib=gio-2.0");
+}