feat(lex_parser): rename functions and update header guard prefix

- Change header guard from `__SMCC_LEX_PARSER_H__` to `__SCC_LEX_PARSER_H__`
- Prefix all lexer functions with `scc_` (e.g., `lex_parse_char` → `scc_lex_parse_char`)
- Add new helper function `scc_lex_parse_is_identifier_header`
- Update references in source and test files to use new function names
- Replace `core_stream_eof` with `scc_stream_eof` for consistency

libs/lex_parser/tests/test_number.c

@@ -9,7 +9,7 @@ cbool check(const char *str, usize expect, usize *output) {
     scc_mem_probe_stream_t mem_stream;
     scc_probe_stream_t *stream =
         scc_mem_probe_stream_init(&mem_stream, str, scc_strlen(str), false);
-    return lex_parse_number(stream, &pos, output);
+    return scc_lex_parse_number(stream, &pos, output);
 }
 
 #define CHECK_VALID(str, expect)                                               \