refactor: remove QBE backend, improve bootstrap compiler and stdlib

Bootstrap compiler improvements:
- Extract findStdlibDir() to remove hardcoded path and deduplicate
- Extract prepareProject()/mergeProject() to share code between cmdCheck/cmdBuild
- Fix C backend to emit warning on unknown types instead of silent 'int'
- Clean up all unused imports across 7 modules
- Makefile: add strip, debug target, clean-all, selfhost strip

QBE removal:
- Remove vendor/qbe/ (74K lines)
- Remove compiler/selfhost/qbe_backend.bux, nim_backend.bux

Stdlib improvements:
- Add Result.bux + Option.bux modules
- Fix Json.bux memory leak (removed double String_Copy)
- Add missing imports in Crypto.bux (Alloc/Free) and Test.bux (PrintLine/PrintInt)
- Clean stale buxc_debug binary

library/std/Channel.bux

@@ -32,7 +32,7 @@ func Channel_Free<T>(ch: *Channel<T>) {
     bux_channel_free(ch.handle);
 }
 
-/* Non-generic wrappers for common types (avoid monomorphization issues) */
+/* Convenience wrappers for common types */
 func Channel_SendInt(ch: *Channel<int>, value: int) {
     bux_channel_send(ch.handle, (&value) as *void);
 }