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