Add TOASTY_WRITE_CREATE_IF_MISSING flag for server-side file auto-creation

Adds support for automatically creating files when write operations target
non-existent files. This is essential for REST PUT operations which should
be able to create new resources.

Implementation:
- Added TOASTY_WRITE_CREATE_IF_MISSING flag to ToastyFS API header
- Updated toasty_write() and toasty_begin_write() to accept flags parameter
- Client sends write flags in MESSAGE_TYPE_WRITE message to metadata server
- Metadata server parses flags and handles file auto-creation atomically:
  * When write fails with FILETREE_NOENT and flag is set
  * Logs creation to WAL for crash consistency
  * Creates file with default 4096-byte chunk size
  * Retries write with newly created file's generation tag
- Updated web proxy PUT handler to use TOASTY_WRITE_CREATE_IF_MISSING
- Updated examples and simulation client for new API signature

Benefits:
- Works for both sync and async APIs
- Single round trip (atomic server-side operation)
- Prevents race conditions
- Proper WAL logging ensures crash consistency

web/DESIGN.txt

@@ -34,9 +34,14 @@ PUT
     ??? (TODO) is returned. If the payload the request is not empty, the
     operation fails with code 400.
 
-    If the path refers to a file, it creates that file with the specified
-    contents. If a file or directory already existed at that path, it is
-    overwritten unless the header "X-ToastyFS-Overwrite: no" is sent.
+    If the path refers to a file, the file is automatically created if it
+    doesn't exist, then the specified contents are written to it. This is
+    implemented using the TOASTY_WRITE_CREATE_IF_MISSING flag which performs
+    file creation and write atomically on the metadata server. Files are
+    created with a default chunk size of 4096 bytes.
+
+    If a file or directory already existed at that path, it is overwritten
+    unless the header "X-ToastyFS-Overwrite: no" is sent.
     (TODO error codes)
 
 DELETE

web/src/proxy_put.c

@@ -7,8 +7,11 @@ bool process_request_put(ProxyState *state, ProxyOperation *operation,
         request->url.path.ptr,
         request->url.path.len,
     };
+    // Use TOASTY_WRITE_CREATE_IF_MISSING to automatically create files
+    // when they don't exist, enabling PUT operations to create new files
     ToastyHandle handle = toasty_begin_write(state->backend, path, 0,
-        request->body.ptr, request->body.len, TOASTY_VERSION_TAG_EMPTY);
+        request->body.ptr, request->body.len, TOASTY_VERSION_TAG_EMPTY,
+        TOASTY_WRITE_CREATE_IF_MISSING);
     if (handle == TOASTY_INVALID) {
         http_response_builder_status(builder, 500); // Internal Server Error
         http_response_builder_send(builder);