Setup retry-hint echoes invoked subcommand (#449)

Previously, running voxtype setup gpu --enable without root suggested retrying as sudo voxtype setup onnx --enable, which is a different command. Now the retry hint matches the subcommand the user invoked, with switch_to deriving the hint from the variant family for the TUI path.

Structural verification

grep -c "Try: sudo voxtype " src/setup/binary.rs
# Expected: 3 (the three error paths)

grep -c "retry_hint" src/setup/binary.rs src/setup/gpu.rs
# Expected: 5+ (param threaded through callers)

Confirm the literal setup onnx --enable hint is no longer hard-coded in binary.rs (it's still in the wrapper script's audit comment, which is intentional: wrappers are only generated for ONNX variants).

grep -c 'Try: sudo voxtype setup onnx' src/setup/binary.rs
# Expected: 0

Runtime tests

These require not having sudo cached and ideally running as a user without write access to /usr/bin/voxtype (e.g., a stock package install).

Path 1: setup gpu

voxtype setup gpu --enable 2>&1 | grep "Try:"
# Expected: Try: sudo voxtype setup gpu --enable

Path 2: setup onnx

voxtype setup onnx --enable 2>&1 | grep "Try:"
# Expected: Try: sudo voxtype setup onnx --enable

Path 3: TUI variant picker

Without sudo cached, pick a Whisper variant in the TUI's General section and observe the on-screen error after pkexec cancellation. The retry hint should be Try: sudo voxtype setup gpu --enable.

Then pick an ONNX variant under the same conditions. The retry hint should be Try: sudo voxtype setup onnx --enable.