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

compiler/bootstrap/c_backend.nim

@@ -1,5 +1,5 @@
-import std/[strformat, strutils, tables]
-import hir, types, token, source_location
+import std/[strformat, strutils]
+import hir, types, token
 
 type
   CBackend* = object
@@ -107,7 +107,12 @@ proc typeToC*(be: var CBackend, typ: Type): string =
     else: return typ.name
   of tkTuple: return "void*"  # TODO: proper tuple struct
   of tkFunc: return "void*"  # TODO: function pointer
-  else: return "int"
+  else:
+    when defined(release):
+      return "void*"
+    else:
+      stderr.writeLine("warning: C backend: unknown type kind " & $typ.kind & ", using void*")
+      return "void*"
 
 proc operatorToC(op: TokenKind): string =
   case op