tune: --freeze-base and --train-data flags silently ignored

[noahgift]

Bug

In tune.rs line 67-68, _freeze_base: bool and _train_data: Option<&Path> are accepted by the CLI but silently discarded via underscore prefix.

Impact

Users who specify --freeze-base to prevent base model weight updates or --train-data to provide training data get no indication these flags are ignored.

Fix

Reject with explicit error since the features are not implemented.