Add TOASTY_WRITE_TRUNCATE_AFTER flag for HTTP PUT semantics

Adds support for truncating files after write operations, enabling proper
HTTP PUT semantics where the entire file content should be replaced.

Implementation:
- Added TOASTY_WRITE_TRUNCATE_AFTER flag (0x02) to ToastyFS API
- Updated file_tree_write() to accept truncate_after parameter:
  * When true, sets file size to exactly offset+length
  * Removes chunks beyond the new file size
  * Marks removed chunks for garbage collection
- Metadata server extracts and passes truncate flag to file_tree_write()
- WAL replay passes false for truncate_after (already handled in original write)
- Web proxy PUT now uses both CREATE_IF_MISSING and TRUNCATE_AFTER flags
- Updated documentation in PROTOCOL.txt, DESIGN.txt, and web/DESIGN.txt

Benefits:
- Proper HTTP PUT semantics (replace entire file content)
- Efficient: single write operation truncates and updates file
- Works with both sync and async APIs
- Garbage collection automatically removes orphaned chunks

misc/PROTOCOL.txt

@@ -122,6 +122,10 @@ the metadata server:
           the metadata server will atomically create it with a default chunk
           size of 4096 bytes and retry the write operation. The creation is
           logged to the WAL for crash consistency.
+        - TOASTY_WRITE_TRUNCATE_AFTER (0x02): Truncate the file after the write
+          operation, setting the file size to exactly offset+length. Any data
+          beyond this point is discarded. Useful for HTTP PUT semantics where
+          the entire file content should be replaced.
 
     The offset and length mark the region that is being written to.