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

tests/parser_test.nim

@@ -111,7 +111,7 @@ suite "Parser":
     check ret.stmtReturnValue.exprCallCallee.kind == ekIdent
 
   test "full sample file":
-    let source = readFile("tests/testdata/sample.bux")
+    let source = readFile(currentSourcePath.parentDir / "testdata" / "sample.bux")
     let lexRes = tokenize(source, "sample.bux")
     check not lexRes.hasErrors
     let parseRes = parse(lexRes.tokens, "sample.bux")