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/wal.c

@@ -473,7 +473,8 @@ int wal_open(WAL *wal, FileTree *file_tree, string file_path, int entry_limit)
             file_tree_delete_entity(file_tree, entry.path, entry.expect_gen);
             break;
         case WAL_ENTRY_WRITE:
-            file_tree_write(file_tree, entry.path, entry.offset, entry.length, entry.num_chunks, entry.expect_gen, &gen, entry.hashes, NULL, NULL);
+            // WAL replay: use false for truncate_after since truncation was already handled
+            file_tree_write(file_tree, entry.path, entry.offset, entry.length, entry.num_chunks, entry.expect_gen, &gen, entry.hashes, NULL, NULL, false);
             break;
         default:
             UNREACHABLE;