Windows support (wip): target layer, Microsoft x64 ABI, portable runtime, CI

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/windows.sh

@@ -0,0 +1,50 @@
+#!/usr/bin/env bash
+# Cross-compiles every test for Windows (x86_64-w64-mingw32) and runs it under Wine.
+# Needs x86_64-w64-mingw32-gcc and wine, e.g.:
+#   nix-shell -p pkgsCross.mingwW64.buildPackages.gcc wine64 --run tests/windows.sh
+# (On a real Windows machine, use tests/run.sh with finch.exe instead.)
+set -u
+cd "$(dirname "$0")"
+FINCH=${FINCH:-../build/finch}
+export WINEDEBUG=-all
+pass=0; failed=0
+tmp=$(mktemp -d)
+trap 'rm -rf "$tmp"' EXIT
+
+run_exe() {  # exe, stdin -> output without Windows' \r
+    wine "$1" < "$2" 2>&1 | tr -d '\r'
+}
+
+for f in run/*.fch; do
+    grep -q "^fn main" "$f" || continue
+    input="${f%.fch}.in"
+    [ -f "$input" ] || input=/dev/null
+    exe="$tmp/$(basename "${f%.fch}").exe"
+    if ! "$FINCH" build "$f" --target windows -o "$exe" > "$tmp/err" 2>&1; then
+        failed=$((failed + 1)); echo "FAIL (build) $f"; head -5 "$tmp/err"; continue
+    fi
+    got=$(run_exe "$exe" "$input")
+    if [ "$got" == "$(cat "${f%.fch}.out")" ]; then
+        pass=$((pass + 1))
+    else
+        failed=$((failed + 1)); echo "FAIL $f"; diff <(echo "$got") "${f%.fch}.out" | head -10
+    fi
+done
+
+for f in fail/*.fch; do
+    expect=$(head -1 "$f" | sed -n 's|^// expect: ||p')
+    exe="$tmp/$(basename "${f%.fch}").exe"
+    if got=$("$FINCH" build "$f" --target windows -o "$exe" 2>&1); then
+        got=$(run_exe "$exe" /dev/null); status=$?
+        status=$( [ -n "$got" ] && echo 1 || echo 0 )
+    else
+        status=1
+    fi
+    if [[ "$got" == *"$expect"* ]]; then
+        pass=$((pass + 1))
+    else
+        failed=$((failed + 1)); echo "FAIL $f (wanted: $expect)"; echo "$got" | head -5
+    fi
+done
+echo "windows (wine): $pass passed, $failed failed"
+[ $failed -eq 0 ]