fix: selfhost CLI run command, lexer char literal, parser test path, calloc in runtime

- Fix lexer unterminated char error test (was treating 'a as lifetime)
- Fix parser test sample.bux path for root-dir execution
- Add bux_system() to runtime.c and run command to selfhost CLI
- Switch bux_alloc from malloc to calloc for zero-init
- Fix module flattening in Cli_Check/Cli_Compile to scan full decl list
- Make test passes fully, selfhost builds successfully

src/lexer.nim

@@ -553,6 +553,8 @@ proc nextToken(lex: var Lexer): Token =
       # Simple heuristic: if peek(2) is ', it's a char literal; else lifetime
       if lex.peek(2) == '\'':
         return lex.scanChar(startLoc, 0)
+      elif lex.peek(2) == '\0':
+        return lex.scanChar(startLoc, 0)
       else:
         # Lifetime: consume ' and then identifier chars
         discard lex.advance()  # '