feat(ast): 更新AST字面量表示方式

更新AST定义以使用词素字符串代替常量值，
并修改AST转储功能以正确显示字面量内容。

BREAKING CHANGE: AST表达式结构体中literal成员从value改为lexme字段。

refactor(pproc): 重构宏展开和文件包含逻辑

将宏展开函数重构为独立接口，实现文件包含处理逻辑，
改进预处理器的状态管理机制。

fix(sstream): 修复文件流初始化错误码返回

修正文件打开失败时的错误码返回值，确保调用方能正确处理异常情况。

libs/pproc/src/pproc_include.c

@@ -0,0 +1,84 @@
+#include <pproc_expand.h>
+#include <scc_pproc.h>
+
+static int switch_file_stack(scc_pproc_t *pp, scc_cstring_t fname,
+                             int is_system) {
+    scc_pproc_file_state_t *file = scc_malloc(sizeof(scc_pproc_file_state_t));
+    Assert(file != null);
+    if (scc_sstream_init(&(file->sstream), fname.data, 1024)) {
+        return -1;
+    }
+    scc_lexer_init(&(file->lexer), scc_sstream_to_ring(&(file->sstream)));
+    file->ring = scc_lexer_to_ring(&(file->lexer), 8, true);
+    pp->cur_ring = file->ring;
+
+    scc_vec_push(pp->file_stack, file);
+    return 0;
+}
+
+void scc_pproc_parse_include(scc_pproc_t *pp) {
+    int ok;
+    scc_lexer_tok_t tok;
+
+    scc_lexer_tok_ring_t *stream = pp->cur_ring;
+    scc_lexer_tok_vec_t org_toks;
+    scc_vec_init(org_toks);
+    while (1) {
+        scc_ring_peek(*stream, tok, ok);
+        if (ok == false)
+            break;
+        scc_ring_next_consume(*stream, tok, ok);
+        scc_vec_push(org_toks, tok);
+        // FIXME endline needed?
+        if (tok.type == SCC_TOK_ENDLINE)
+            break;
+    }
+
+    scc_lexer_tok_ring_t out_ring;
+    scc_pproc_expand_by_vec(&pp->macro_table, &org_toks, &out_ring);
+    scc_cstring_t line = scc_cstring_create();
+    while (1) {
+        scc_ring_next_consume(out_ring, tok, ok);
+        if (!ok)
+            break;
+        if (scc_get_tok_subtype(tok.type) != SCC_TOK_SUBTYPE_EMPTYSPACE &&
+            scc_get_tok_subtype(tok.type) != SCC_TOK_SUBTYPE_COMMENT) {
+            scc_cstring_append(&line, &tok.lexeme);
+        }
+        scc_lexer_tok_drop(&tok);
+    }
+    scc_ring_free(out_ring);
+
+    const char *includename = scc_cstring_as_cstr(&line);
+    int len = scc_cstring_len(&line);
+    if (len < 2) {
+        goto ERROR;
+    } else if (len == 2) {
+        goto ERROR;
+    } else {
+        if (includename[0] == '\"') {
+            if (includename[len - 1] != '\"') {
+                goto ERROR;
+            }
+        } else if (includename[0] == '<') {
+            if (includename[len - 1] != '>') {
+                goto ERROR;
+            }
+        } else {
+            goto ERROR;
+        }
+    }
+    scc_cstring_t fname = scc_cstring_create();
+    for (int i = 1; i < len - 1; i++) {
+        scc_cstring_append_ch(&fname, includename[i]);
+    }
+    scc_cstring_free(&line);
+    int is_system = includename[0] == '<';
+    if (switch_file_stack(pp, fname, is_system)) {
+        goto ERROR;
+    }
+    return;
+ERROR:
+    LOG_ERROR("Invalid include filename need \"FILENAME\" or <FILENAME>");
+    scc_cstring_free(&line);
+}