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/Crypto.bux

@@ -1,4 +1,6 @@
 module Std::Crypto {
+import Std::Mem::{Alloc, Free};
+import Std::String::{String_Len};
 extern func bux_sha256(data: String, len: int, out: *void);
 extern func bux_hmac_sha256(key: String, keylen: int, msg: String, msglen: int, out: *void);
 extern func bux_random_bytes(buf: *void, len: int) -> int;