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

README.md

@@ -151,7 +151,7 @@ Pick the guide that fits you:
 - [x] A standard library in Finch: `json`, `path`, `time`, `proc`, `fs`
 - [x] `const`, `if` as a value, `"\(x)"` in text, `"""` text, C unions and `#define` expressions
 - [x] `test` blocks and `finch test`, `finch fmt`, `finch.toml`, a build cache, the chain of calls on runtime errors
-- [x] Threads (`import thread`) and atomics; C++ files; `--target android` (built, not yet tried on a device)
+- [x] Threads (`import thread`) and atomics; C++ files; `--target android` (compiles; NDK linking and devices untried)
 
 ## Layout