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/metadata_server.c

@@ -634,13 +634,17 @@ process_client_write(MetadataServer *state, int conn_idx, ByteView msg)
         assert(0); // TODO
     }
 
+    // Extract flag values
+    #define TOASTY_WRITE_CREATE_IF_MISSING (1 << 0)
+    #define TOASTY_WRITE_TRUNCATE_AFTER    (1 << 1)
+    bool truncate_after = (flags & TOASTY_WRITE_TRUNCATE_AFTER) != 0;
+
     uint64_t new_gen;
     int ret = file_tree_write(&state->file_tree, path, offset, length,
-        num_chunks, expect_gen, &new_gen, new_hashes, removed_hashes, &num_removed);
+        num_chunks, expect_gen, &new_gen, new_hashes, removed_hashes, &num_removed, truncate_after);
 
     // If write failed because file doesn't exist and CREATE_IF_MISSING flag is set,
     // create the file and retry the write
-    #define TOASTY_WRITE_CREATE_IF_MISSING (1 << 0)
     if (ret == FILETREE_NOENT && (flags & TOASTY_WRITE_CREATE_IF_MISSING)) {
         // Create the file with default chunk size of 4096 bytes
         uint64_t chunk_size = 4096;
@@ -656,7 +660,7 @@ process_client_write(MetadataServer *state, int conn_idx, ByteView msg)
         if (create_ret == 0) {
             // File created successfully, retry the write with the new generation
             ret = file_tree_write(&state->file_tree, path, offset, length,
-                num_chunks, create_gen, &new_gen, new_hashes, removed_hashes, &num_removed);
+                num_chunks, create_gen, &new_gen, new_hashes, removed_hashes, &num_removed, truncate_after);
         }
     }