feat(wasm): port Ghostty's wasm32 build

Ports main_wasm.zig as crates/ghostty-wasm (cdylib exporting the Zig
names: atlas_*, face_*, deferred_face_*, shaper_*, config_*), the
web_canvas font face and shaper (backend selected on wasm32), os/wasm
(logging, target, JS interop) and config/Wasm.zig.

C-library dependencies (freetype, harfbuzz, fontconfig, onig, gettext,
D-Bus, rustix) are target-gated. Like Zig's wasm build, link regex
search (StringMap::search_iterator) is unavailable without Oniguruma;
Collection/SharedGrid/harfbuzz are compiled out on wasm32. ghostty-os
switches from forbid to deny unsafe_code for the JS import blocks.

The JS import ABI is currently a documented handle-based interface with
a reference host (js/ghostty_host.js); aligning it with zig-js's exact
import names follows separately.

Test Plan:
- cargo build --target wasm32-unknown-unknown -p ghostty-font \
  -p ghostty-config -p ghostty-terminal -p ghostty-vt -p ghostty-wasm
- cargo test -p ghostty-wasm (node runs the module against a fake canvas)

crates/ghostty-wasm/tests/node.rs

@@ -0,0 +1,60 @@
+//! Builds the wasm module and runs `tests/node_test.js` against it with
+//! node, using stub JS imports (a fake canvas measuring API). Skipped when
+//! node or the `wasm32-unknown-unknown` target is not available.
+
+use std::{path::PathBuf, process::Command};
+
+#[test]
+fn wasm_module_runs_under_node() {
+    if Command::new("node").arg("--version").output().is_err() {
+        eprintln!("skipping: node not found");
+        return;
+    }
+
+    let manifest = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    let target_dir = std::env::var_os("CARGO_TARGET_DIR")
+        .map_or_else(|| manifest.join("../../target"), PathBuf::from);
+    // A separate directory so this does not contend with the outer build.
+    let target_dir = target_dir.join("wasm-test");
+
+    let cargo = std::env::var("CARGO").unwrap_or_else(|_| "cargo".to_owned());
+    let out = Command::new(cargo)
+        .args([
+            "build",
+            "--target",
+            "wasm32-unknown-unknown",
+            "-p",
+            "ghostty-wasm",
+            "--manifest-path",
+        ])
+        .arg(manifest.join("Cargo.toml"))
+        .env("CARGO_TARGET_DIR", &target_dir)
+        .output()
+        .expect("run cargo");
+    if !out.status.success() {
+        let stderr = String::from_utf8_lossy(&out.stderr);
+        if stderr.contains("can't find crate for `std`")
+            || stderr.contains("target may not be installed")
+        {
+            eprintln!("skipping: wasm32-unknown-unknown target not installed");
+            return;
+        }
+        panic!("wasm build failed:\n{stderr}");
+    }
+
+    let wasm = target_dir.join("wasm32-unknown-unknown/debug/ghostty_wasm.wasm");
+    assert!(wasm.exists(), "missing {}", wasm.display());
+
+    let out = Command::new("node")
+        .arg(manifest.join("tests/node_test.js"))
+        .arg(&wasm)
+        .output()
+        .expect("run node");
+    assert!(
+        out.status.success(),
+        "node test failed:\nstdout: {}\nstderr: {}",
+        String::from_utf8_lossy(&out.stdout),
+        String::from_utf8_lossy(&out.stderr)
+    );
+    assert!(String::from_utf8_lossy(&out.stdout).contains("ok"));
+}