feat: const 编译期常量 — const N = 42;

Token(77): +TOK_CONST, AST_LET_STMT 新增 is_const, Symbol 新增 const_value
sema: 字面量初始化自动折叠为编译期常量值

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/lexer/token.c

@@ -5,7 +5,7 @@
 #include <inttypes.h>
 
 static const char* NAMES[] = {
-    [TOK_FN] = "fn", [TOK_LET] = "let", [TOK_VAR] = "var", [TOK_IF] = "if", [TOK_GUARD] = "guard",
+    [TOK_FN] = "fn", [TOK_LET] = "let", [TOK_VAR] = "var", [TOK_CONST] = "const", [TOK_IF] = "if", [TOK_GUARD] = "guard",
     [TOK_PUB] = "pub", [TOK_MOD] = "mod", [TOK_USE] = "use",
     [TOK_TRAIT] = "trait", [TOK_SELF] = "Self",
     [TOK_ELSE] = "else", [TOK_WHILE] = "while", [TOK_FOR] = "for", [TOK_IN] = "in", [TOK_RETURN] = "return",