selfhost: fix parser infinite loop + lexer token limit

- Parser_Parse: add beforePos safeguard to skip unrecognized tokens
  instead of looping forever when parserParseDecl returns null
- Lexer: increase maxTokens 4096 -> 32768; add explicit break on limit
- CLI: null-check ReadFile result before String_Eq
- Nim CLI: add -O2 to cc invocation for faster self-hosted builds

src_bux/cli.bux

@@ -106,7 +106,7 @@ func Cli_Compile(source: String, sourceName: String) -> String {
 
 func Cli_Build(srcPath: String, outPath: String) -> int {
     let source: String = ReadFile(srcPath);
-    if String_Eq(source, "") || !FileExists(srcPath) {
+    if source == null as String || String_Eq(source, "") || !FileExists(srcPath) {
         Print("Error: cannot read source file: ");
         PrintLine(srcPath);
         return 1;
@@ -142,7 +142,7 @@ func Cli_Check(srcPath: String) -> int {
     Print("Check: ");
     PrintLine(srcPath);
     let source: String = ReadFile(srcPath);
-    if String_Eq(source, "") {
+    if source == null as String || String_Eq(source, "") {
         PrintLine("Error: cannot read source file");
         return 1;
     }