refactor(ir): 将IR节点重构为值引用并添加字符串字面量支持

- 将scc_ir_node_ref_t重命名为scc_ir_value_ref_t，并更新所有相关API
- 修改IR定义中的节点标签枚举为值标签枚举(scc_ir_node_tag_t -> scc_ir_value_tag_t)
- 更新AST到IR转换器中所有节点引用的类型声明
- 添加对内置类型(VOID, CHAR, INT等)的完整映射实现
- 实现字符串字面量的常量数组创建功能
- 更新项目名称从"Simple Modual C Compiler"为"Simple C Compiler"
- 在Doxyfile中排除external目录的文档生成
- 移除未使用的strpool字段并重命名decl到IR的映射表

BREAKING CHANGE: IR节点引用类型已更改，所有使用scc_ir_node_ref_t的地方需
替换为scc_ir_value_ref_t。

Doxyfile

@@ -42,7 +42,7 @@ DOXYFILE_ENCODING      = UTF-8
 # title of most generated pages and in a few other places.
 # The default value is: My Project.
 
-PROJECT_NAME           = "Simple Modual C Compiler"
+PROJECT_NAME           = "Simple C Compiler"
 
 # The PROJECT_NUMBER tag can be used to enter a project or revision number. This
 # could be handy for archiving the generated documentation or if some version
@@ -1112,7 +1112,7 @@ EXCLUDE_SYMLINKS       = NO
 # Note that the wildcards are matched against the file with absolute path, so to
 # exclude all test directories for example use the pattern */test/*
 
-EXCLUDE_PATTERNS       =
+EXCLUDE_PATTERNS       = */external/*
 
 # The EXCLUDE_SYMBOLS tag can be used to specify one or more symbol names
 # (namespaces, classes, functions, etc.) that should be excluded from the