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

scripts/windows-crosscheck.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+# Compile-check (clippy -D warnings) the Windows code paths from Linux with
+# mingw. No linking happens, so a stub fontconfig.pc is enough. See
+# docs/rewrite/notes/windows-WIN.md.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+W="$PWD/target/windows-cross"
+mkdir -p "$W/pkgconfig"
+cat > "$W/pkgconfig/fontconfig.pc" <<'EOS'
+Name: fontconfig
+Description: stub for cross cargo check
+Version: 2.15.0
+Libs: -lfontconfig
+Cflags:
+EOS
+export RUSTC_WRAPPER= CARGO_BUILD_RUSTC_WRAPPER= PKG_CONFIG_ALLOW_CROSS=1 PKG_CONFIG_PATH="$W/pkgconfig"
+export CC_x86_64_pc_windows_gnu=x86_64-w64-mingw32-gcc CXX_x86_64_pc_windows_gnu=x86_64-w64-mingw32-g++ \
+  AR_x86_64_pc_windows_gnu=x86_64-w64-mingw32-ar
+cargo clippy --target x86_64-pc-windows-gnu --lib --tests \
+  -p ghostty-vt -p ghostty-unicode -p ghostty-datastruct -p ghostty-input -p ghostty-os \
+  -p ghostty-config -p ghostty-terminal -p ghostty-termio -p ghostty-crash -p ghostty-core \
+  -p ghostty-cli -- -D warnings