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/wasm-check.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# Build + clippy the wasm32 targets (Ghostty's wasm build and libghostty-vt
+# for wasm), then run the node-based wasm tests.
+set -euo pipefail
+cd "$(dirname "$0")/.."
+cargo clippy --target wasm32-unknown-unknown -p ghostty-wasm -p ghostty-vt-capi \
+  -p ghostty-font -p ghostty-config -p ghostty-terminal -p ghostty-vt --lib -- -D warnings
+cargo build --target wasm32-unknown-unknown --release -p ghostty-wasm -p ghostty-vt-capi
+cargo test -p ghostty-wasm -p ghostty-vt-capi --test node --test wasm_examples