feat: expose ordered render command prefix

[PORTALSURFER]

Goal\n\nProvide an additive Vello rendering hook that lets a host place one caller-owned command buffer immediately before Vello internal render work in the same queue submission.\n\n## Scope\n\n- Add Renderer::render_to_texture_with_command_buffer with an optional wgpu CommandBuffer prefix.\n- Keep Renderer::render_to_texture signature-compatible and preserve its existing no-prefix path.\n- Submit prefix then Vello render commands through one Queue::submit call.\n- Add focused deterministic coverage for prefix ordering and the no-prefix fallback.\n- Document ownership and failure behavior.\n\n## Non-goals\n\n- No wgpu-profiler changes.\n- No requirement for TIMESTAMP_QUERY_INSIDE_PASSES.\n- No GPU timing policy, readback, or presentation behavior.\n- No Radiant source changes; this is the dependency prerequisite for a later host integration.\n\n## Definition of done\n\n- Existing callers retain the same API and behavior.\n- A supplied prefix is consumed and submitted exactly once before Vello work.\n- Recording failures never submit the prefix independently.\n- No-prefix callers retain the prior single-buffer submission path.\n- Focused tests and the project checks pass.\n\n## Validation and results\n\n- cargo fmt --all -- --check\n- cargo check -p vello --locked\n- cargo check -p vello --all-features --locked\n- cargo check -p vello --no-default-features --locked\n- cargo check -p vello --features wgpu-profiler --locked\n- cargo test -p vello --all-features --lib --locked\n- cargo test -p vello --doc --locked\n- cargo test -p vello_tests --no-run --locked\n- cargo clippy -p vello --all-targets --all-features --locked\n- git diff --check\n- Existing filled_square_gpu smoke passed.\n\nOnly pre-existing lint warnings were emitted. No GPU runtime integration was added because this hook only establishes queue submission ordering.\n\n## Known limitations\n\nRadiant currently consumes Vello 0.9.0. The follow-up Radiant producer will use this hook after an upstream Vello release containing it.\n\nPlease do not merge until PORTALSURFER explicitly approves the reviewed PR head.