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

web/DESIGN.txt

@@ -35,10 +35,13 @@ PUT
     operation fails with code 400.
 
     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.
+    doesn't exist, then the specified contents are written to it, replacing
+    the entire file content (HTTP PUT semantics). This is implemented using
+    two flags:
+    - TOASTY_WRITE_CREATE_IF_MISSING: Creates file if it doesn't exist
+    - TOASTY_WRITE_TRUNCATE_AFTER: Truncates file after the write, discarding
+      any data beyond the uploaded content
+    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.