feat: validate bounds for --llm-rate, --cull-keep, --image-long-side

[rasros]

What

Adds range validation for three numeric CLI flags that previously accepted nonsensical values and silently produced degenerate behavior.

• --llm-rate must be in [0.0, 1.0] (it's a probability). Out-of-range explicit values now error instead of being passed through.
• --cull-keep must be in (0.0, 1.0]. 0 kept no beams eligible for expansion (stalling beam search); > 1.0 was meaningless.
• --image-long-side must be > 0. The old check only rejected negatives, so 0 slipped through into a degenerate downscale. The message is now specific instead of the generic "Configuration values cannot be negative".

Tests

Added parametrized cases for each bound (in/out of range) in test_cli.py. Full suite: 356 passed.