Update tokenstream_open to accept Buffer parameter instead of file path

The tokenstream_open function now takes a Buffer* parameter instead of a file
path string, making the API more flexible and allowing the caller to manage
buffer lifetime. The tokenstream_close function continues to close the underlying
buffer as documented.

- Changed tokenstream_open signature from (const char* path) to (Buffer* buffer)
- Updated implementation to accept and use the provided buffer directly
- Updated all tests to open buffers separately and pass them to tokenstream_open
- Added #include "buffer.h" to token.h for Buffer type definition
- All 15 tests pass

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

v0/token.h

@@ -4,6 +4,8 @@
 #ifndef TOKEN_H
 #define TOKEN_H
 
+#include "buffer.h"
+
 typedef enum {
 	// Keywords
 	TOKEN_MODULE,
@@ -34,11 +36,14 @@ typedef struct TokenStream TokenStream;
 const char* token_to_string(Token token);
 
 /**
- * Opens a file and returns a TokenStream for it.
- * @param path The path to the file.
+ * Returns a TokenStream for a given buffer.
+ * 
+ * When the tokenstream is closed, the underlying buffer is also closed.
+ * 
+ * @param buffer The buffer to read from.
  * @returns A handle to the TokenStream.
  */
-TokenStream* tokenstream_open(const char* path);
+TokenStream* tokenstream_open(Buffer* buffer);
 
 /**
  * Closes a TokenStream.