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-os/src/unsupported.rs

@@ -0,0 +1,104 @@
+//! API-compatible stand-ins for OS facilities that do not exist on wasm32,
+//! so crates that are also built for wasm (`ghostty-config`) compile
+//! unchanged. In Zig these are simply never analyzed on wasm
+//! (`os/passwd.zig` even `@compileError`s); here each reports "unavailable"
+//! the way the Zig callers' error paths expect.
+
+use std::io;
+
+/// `homedir.zig` is unavailable: nothing can be expanded.
+pub mod homedir {
+    #[derive(Debug, thiserror::Error, PartialEq, Eq)]
+    pub enum ExpandError {
+        #[error("home detection failed")]
+        HomeDetectionFailed,
+        #[error("buffer too small")]
+        BufferTooSmall,
+    }
+
+    /// Paths without a leading `~/` pass through; others fail.
+    pub fn expand_home(path: &str) -> Result<String, ExpandError> {
+        if path.starts_with("~/") {
+            Err(ExpandError::HomeDetectionFailed)
+        } else {
+            Ok(path.to_owned())
+        }
+    }
+}
+
+pub mod passwd {
+    #[derive(Debug, Clone, Default, PartialEq, Eq)]
+    pub struct Entry {
+        pub shell: Option<String>,
+        pub home: Option<String>,
+        pub name: Option<String>,
+    }
+
+    /// Zig: `@compileError("passwd is not available for wasm")`.
+    pub fn get() -> std::io::Result<Entry> {
+        Err(std::io::Error::new(
+            std::io::ErrorKind::Unsupported,
+            "passwd is not available for wasm",
+        ))
+    }
+}
+
+pub mod xdg {
+    use std::ffi::CStr;
+
+    #[derive(Debug, Clone, Copy, Default)]
+    pub struct Options<'a> {
+        pub subdir: Option<&'a str>,
+        pub home: Option<&'a str>,
+    }
+
+    #[derive(Debug, thiserror::Error, PartialEq, Eq)]
+    pub enum Error {
+        #[error("xdg directories are not available on wasm")]
+        Unavailable,
+    }
+
+    pub fn config(_: &Options<'_>) -> Result<String, Error> {
+        Err(Error::Unavailable)
+    }
+
+    pub fn cache(_: &Options<'_>) -> Result<String, Error> {
+        Err(Error::Unavailable)
+    }
+
+    pub fn state(_: &Options<'_>) -> Result<String, Error> {
+        Err(Error::Unavailable)
+    }
+
+    /// No `xdg-terminal-exec` mode on wasm.
+    pub fn parse_terminal_exec<'a>(_: &'a [&'a CStr]) -> Option<&'a [&'a CStr]> {
+        None
+    }
+}
+
+#[derive(Debug, Clone, Default, PartialEq, Eq)]
+pub struct ResourcesDir {
+    pub app_path: Option<String>,
+    pub host_path: Option<String>,
+}
+
+impl ResourcesDir {
+    pub fn app(&self) -> Option<&str> {
+        self.app_path.as_deref()
+    }
+
+    pub fn host(&self) -> Option<&str> {
+        self.host_path.as_deref()
+    }
+}
+
+pub fn resources_dir() -> io::Result<ResourcesDir> {
+    Err(io::Error::new(
+        io::ErrorKind::Unsupported,
+        "resources dir is not available on wasm",
+    ))
+}
+
+pub fn is_flatpak() -> bool {
+    false
+}