build: add reproducible macOS, Windows and wasm cross-check scripts

The non-Linux code paths can only be compile-checked on this machine;
agents had used ad-hoc wrapper setups. These scripts make the checks
reproducible and are wired into `just check-macos`, `check-windows`,
`check-wasm` and `check-all`:
- macos-crosscheck.sh: zig cc wrappers (dropping -arch/-mmacosx-
  version-min) plus a declaration-only os/log.h stub so the few C build
  scripts compile; clippy -D warnings on all 17 cross-platform crates
  for aarch64/x86_64-apple-darwin.
- windows-crosscheck.sh: mingw toolchain and a stub fontconfig.pc;
  clippy -D warnings for x86_64-pc-windows-gnu.
- wasm-check.sh: wasm32 clippy/build and the node-based wasm tests.

Also fixes a needless_pass_by_value lint in osbytes' non-Unix branch
that only the Windows check surfaces.

Test Plan:
- just check-macos; just check-macos x86_64-apple-darwin
- just check-windows; just check-wasm

crates/ghostty-os/src/osbytes.rs

@@ -84,7 +84,11 @@ pub fn os_from_bytes(b: Vec<u8>) -> OsString {
 
 #[cfg(not(unix))]
 pub fn os_from_bytes(b: Vec<u8>) -> OsString {
-    OsString::from(String::from_utf8_lossy(&b).into_owned())
+    // Windows OS strings are WTF-16; valid UTF-8 converts losslessly and is
+    // reused without copying, anything else is replaced like Zig's utf8ToUtf16.
+    OsString::from(
+        String::from_utf8(b).unwrap_or_else(|e| String::from_utf8_lossy(e.as_bytes()).into_owned()),
+    )
 }
 
 /// OS string to reversible `String` (see the module docs).