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/src/proxy_put.c

@@ -7,11 +7,12 @@ 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
+    // Use both flags for proper HTTP PUT semantics:
+    // - TOASTY_WRITE_CREATE_IF_MISSING: Create file if it doesn't exist
+    // - TOASTY_WRITE_TRUNCATE_AFTER: Replace entire file content (truncate after write)
     ToastyHandle handle = toasty_begin_write(state->backend, path, 0,
         request->body.ptr, request->body.len, TOASTY_VERSION_TAG_EMPTY,
-        TOASTY_WRITE_CREATE_IF_MISSING);
+        TOASTY_WRITE_CREATE_IF_MISSING | TOASTY_WRITE_TRUNCATE_AFTER);
     if (handle == TOASTY_INVALID) {
         http_response_builder_status(builder, 500); // Internal Server Error
         http_response_builder_send(builder);