Warnings for big copies: a struct with 256+ bytes of plain data as a parameter or in for x in list

- on stderr (finch ir stays the IR), as yellow warnings in editors (@W in the index), once per place
- str, arrays and maps inside a struct don't count: passing it copies only their handles
- none in the compiler, the examples or the tests other than the new one
- docs: the warning instead of "not planned"; what was and wasn't checked for Android

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016GzNP6r8dg6WDxUVZy8wjL

tests/cross.sh

@@ -34,7 +34,8 @@ for f in run/*.fch new/*.fch; do
         failed=$((failed + 1)); echo "FAIL (build) $f"; head -5 "$tmp/err"; continue
     fi
     got=$(run_exe "$exe" "$input")
-    if [ "$got" == "$(tr -d '\r' < "${f%.fch}.out")" ]; then
+    # (warnings come from the build, which is not part of what the program prints)
+    if [ "$got" == "$(tr -d '\r' < "${f%.fch}.out" | grep -v ': warning: ')" ]; then
         pass=$((pass + 1))
     else
         failed=$((failed + 1)); echo "FAIL $f"; diff <(echo "$got") "${f%.fch}.out" | head -10