feat(fleet): add bootstrap and down verbs

[hari (harivansh-afk)]

feat(fleet): add bootstrap and down verbs

ENG-1605 needs deploy-test to reuse the generic fleet create and cleanup path without invoking replacement-image pushes or switches. This adds ix-fleet bootstrap, which creates selected nodes from their bootstrap image, waits for node readiness, and preserves dependency ordering while allowing independent nodes in a batch to run concurrently.

This also adds ix-fleet down, which removes selected nodes in reverse plan order, treats already-missing nodes as absent, and continues cleanup across nodes before returning one failure if any removal failed.

Validation:

• nix shell --impure --expr 'with import {}; python313.withPackages (ps: [ ps.pydantic ])' -c env PYTHONPATH=packages/ix-fleet/src python -m unittest packages/ix-fleet/tests/test_ix_fleet.py
• nix eval .#checks.x86_64-linux.eval.name

Note: nix build .#checks.x86_64-linux.eval cannot run on this aarch64 host because Nix refuses to build the x86_64 cargo vendor derivation locally.