test: add missing graph/facet-filter browser baseline

Follow-up to 1b3a5e8a. The graph/facet-filter.jpg baseline was
deleted as "captured against polluted state" during the pinchtab
drift fix, but never regenerated against the clean post-profile-nuke
state — leaving `make review` browser-verification at 18/19 on a
fresh checkout. Regenerates the file so the scenario has a
first-time baseline like graph/smoke, graph/load, observer/smoke.

Baselines are for human review (no pixel comparison in the gate);
this file exists purely so `verify_browser.py` stops erroring with
"baseline not found".

Refs req_7yqrvikr.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>