feat: for 循环 + range (for i in start..end)

- lexer: TOK_FOR, TOK_IN, TOK_DOT_DOT + 修复数字中 .. 解析
- parser: for-in-range → desugar to {let mut, while, assign}
- 零 sema/codegen 改动，复用现有机制
- 新增 2 个集成测试 (10_for_range.l, 11_for_step2.l)
- mem2reg: LLVM 22 C API 不导出，标注已知限制

基于 Codex 分析报告 §6 P0 #3。

src/lexer/lexer.c

@@ -45,7 +45,7 @@ static Token lex_number(Lexer* l) {
     int start = l->pos;
     TokenKind kind = TOK_INT_LIT;
     while (isdigit(peek(l))) advance(l);
-    if (peek(l) == '.') {
+    if (peek(l) == '.' && peek_next(l) != '.') {
         kind = TOK_FLOAT_LIT; advance(l);
         while (isdigit(peek(l))) advance(l);
     }
@@ -57,7 +57,8 @@ static TokenKind check_keyword(const Token* tok) {
     KW("fn",     TOK_FN);     KW("let",    TOK_LET);
     KW("mut",    TOK_MUT);
     KW("if",     TOK_IF);     KW("else",   TOK_ELSE);
-    KW("while",  TOK_WHILE);  KW("return", TOK_RETURN);
+    KW("while",  TOK_WHILE);  KW("for",    TOK_FOR);  KW("in",     TOK_IN);
+	    KW("return", TOK_RETURN);
     KW("i64",    TOK_I64);    KW("f64",    TOK_F64);
     KW("bool",   TOK_BOOL);   KW("str",    TOK_STR);
     KW("void",   TOK_VOID);
@@ -124,6 +125,7 @@ Token* lex(Arena* a, const char* source, const char* filename,
         else if (c == '>') { tokens[idx++] = make_token(&l, TOK_GT, l.pos, 1); advance(&l); }
         else if (c == '&' && peek_next(&l) == '&') { tokens[idx++] = make_token(&l, TOK_AND_AND, l.pos, 2); advance(&l); advance(&l); }
         else if (c == '|' && peek_next(&l) == '|') { tokens[idx++] = make_token(&l, TOK_PIPE_PIPE, l.pos, 2); advance(&l); advance(&l); }
+        else if (c == '.' && peek_next(&l) == '.') { tokens[idx++] = make_token(&l, TOK_DOT_DOT, l.pos, 2); advance(&l); advance(&l); }
         else if (c == '(') { tokens[idx++] = make_token(&l, TOK_LPAREN, l.pos, 1); advance(&l); }
         else if (c == ')') { tokens[idx++] = make_token(&l, TOK_RPAREN, l.pos, 1); advance(&l); }
         else if (c == '{') { tokens[idx++] = make_token(&l, TOK_LBRACE, l.pos, 1); advance(&l); }