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

_selfhost/src/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;
     }

_selfhost/src/lexer.bux

@@ -42,7 +42,7 @@ func Lex_IsIdentChar(c: uint32) -> bool {
 // Lexer state
 // ---------------------------------------------------------------------------
 
-const maxTokens: int = 4096;
+const maxTokens: int = 32768;
 const maxDiags: int = 128;
 
 struct LexerDiag {
@@ -587,7 +587,6 @@ func lexNextToken(lex: *Lexer) {
         lexEmitToken(lex, tkEndOfFile);
         return;
     }
-
     let c: uint32 = lexPeek(lex, 0);
 
     if c == 10 { // \n
@@ -673,6 +672,10 @@ func Lexer_Tokenize(source: String) -> *Lexer {
 
     while true {
         lexNextToken(lex);
+        if lex.tokenCount >= maxTokens {
+            lexEmitDiag(lex, "too many tokens");
+            break;
+        }
         if lex.tokens[lex.tokenCount - 1].kind == tkEndOfFile {
             break;
         }

_selfhost/src/parser.bux

@@ -1195,12 +1195,17 @@ func Parser_Parse(tokens: *LexToken, tokenCount: int) -> *Module {
             discard parserAdvance(p);
             continue;
         }
+        let beforePos: int = p.pos;
         let decl: *Decl = parserParseDecl(p);
         if decl != null as *Decl {
             decl.childDecl2 = mod.firstItem; // push front
             mod.firstItem = decl;
             mod.itemCount = mod.itemCount + 1;
         }
+        // Infinite-loop safeguard: if no progress, skip token
+        if p.pos == beforePos {
+            discard parserAdvance(p);
+        }
     }
 
     return mod;