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

src/file_tree.h

@@ -77,7 +77,8 @@ int file_tree_write(FileTree *ft, string path,
     uint64_t *new_gen,
     SHA256 *hashes,
     SHA256 *removed_hashes,
-    int *num_removed);
+    int *num_removed,
+    bool truncate_after);
 
 int file_tree_read(FileTree *ft, string path,
     uint64_t off, uint64_t len, uint64_t *gen, uint64_t *chunk_size,