fix(commands): allow bufdo to delete buffers

[fcoury-oai]

PR #311 added :bufdo, but deleting buffers remained outside its supported command subset. As a result, the common Vim workflow :bufdo bd reported an unsupported-command error even though traversal already snapshots stable buffer IDs.

Allow DeleteBuffer during :bufdo traversal. Each snapshotted buffer is deleted once, and the replacement blank buffer created after deleting the final target is not revisited. An unforced :bufdo bd stops at the first dirty buffer and preserves the existing unsaved-change error; :bufdo bd! discards dirty buffers and continues.

The command architecture notes now document deletion semantics, and focused integration coverage exercises clean traversal, stop-on-dirty behavior, and forced deletion.

How to Test

1. Open several clean buffers and run :bufdo bd. Confirm every original buffer is deleted and Red leaves one fresh [No Name] buffer.
2. Open several buffers, modify one without saving, and run :bufdo bd. Confirm traversal stops on the dirty buffer with No write since last change (add ! to override) and does not delete later buffers.
3. Run :bufdo bd!. Confirm the remaining buffers are deleted and Red leaves one fresh [No Name] buffer.
4. Run cargo test --test editing bufdo_. All five focused bufdo integration tests should pass, including bufdo_deletes_each_snapshotted_buffer_and_leaves_a_blank_buffer and bufdo_delete_stops_at_a_dirty_buffer_unless_forced.