fix(golden): refresh knob_step reference

[j0sh]

This requires an updated knob_step.tar.gz uploaded to Huggingface; not sure I have access to that

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Summary

Refresh the knob_step golden reference from current DEMON main
and tighten its thresholds around the new canonical.

Also document that refs.json's per-scenario env block is
provenance only, not part of the golden pass/fail contract.

What Changed

• Replaced the knob_step reference bundle hash and canonical hash in
  tests/golden/refs.json
• Recalibrated knob_step thresholds for the refreshed canonical:
  mel_l2 0.08, rms_db_diff 0.1, win_cos_min 0.999
• Updated the threshold note to explain the 2026-06-25 refresh and the
  observed same-pod / fresh-pod verification spread
• Normalized the recorded pod URL to remove the signed query token
• Added a README note clarifying that refs.json env metadata is
  provenance only and may be sparse for queued production-pod captures

Why

The old 2026-06-04 knob_step canonical had drifted enough that clean
current runs were failing the calibrated mel_l2 <= 0.13 gate.

A fresh reference captured from DEMON main fixes that mismatch.
Same-pod repeat-3 noise remained tiny, while fresh-pod verification
runs against the new canonical stayed comfortably inside the updated
thresholds.

Verification

• Reproduced the old failure against live pods before the refresh
• Captured a fresh knob_step canonical from DEMON main
• Ran a same-pod repeat-3 variance probe
• Verified fresh-pod golden runs pass against the refreshed canonical