feat(smurf_util): Add keep_channels_subset helper

[ruck314]

Summary

• Adds SmurfUtilMixin.keep_channels_subset(band, subbands=None, freq_min_mhz=None, freq_max_mhz=None, channels_to_keep=None) in python/pysmurf/client/util/smurf_util.py, sitting next to band_off / channel_off.
• Disables currently-on channels (per which_on) that fail any supplied filter; filters combine with logical AND; no-op when no filters are supplied.
• Composes existing helpers (which_on, get_subband_from_channel, channel_to_freq, channel_off); no new low-level API introduced.
• Mirrors the caller-supplies-the-metric pattern of turn_off_noisy_channels and check_lock.

Closes #27.

Usage

The three examples from the original issue:

S.keep_channels_subset(band, subbands=range(13, 16))               # subbands 13-15
S.keep_channels_subset(band, freq_min_mhz=5100, freq_max_mhz=5200) # 5.1-5.2 GHz
S.keep_channels_subset(band, channels_to_keep=tes_like_chs)        # external IV/TES filter

The IV/TES-quality criterion is intentionally left to the caller — analyze_iv already returns metrics, so policy (which R_n / p_trans is "TES-like") stays in calling code:

iv = S.analyze_iv_from_file(...)
good = [ch for ch in iv[band] if iv[band][ch]['R_n'] > 5e-3]
S.keep_channels_subset(band, channels_to_keep=good)

Test plan

• flake8 --count python/ clean (matches CI)
• AST parse confirms signature, @set_action() decorator, and docstring
• python -m py_compile clean
• Hardware smoke test: tune a band, call S.keep_channels_subset(band, subbands=range(13, 16)), verify S.which_on(band) afterward returns only channels whose S.get_subband_from_channel(band, ch) is in {13, 14, 15}
• Hardware smoke test: cross-check freq_min_mhz / freq_max_mhz against S.channel_to_freq(band)
• Confirm band_off / channel_off behavior unchanged