viz: fix Pages CI stale check + minify index.html

Pages workflow rejected previous commit because docs/data/taxonomy.json
contained a wall-clock generated_at timestamp that re-rebuilt on every
CI run, defeating the idempotency check. Remove the timestamp — git
commit date is the canonical generation time.

- scripts/build_viz_data.py: drop generated_at field (and datetime
  imports that became unused)
- docs/data/taxonomy.json: regenerated without generated_at; n_assets,
  n_active_sub_sectors, snapshot_date preserved
- docs/index.html: minified inline CSS + removed source comments
  (committed from QD-F's local trim; functional verified)

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

Author: quantbai

docs/data/taxonomy.json

@@ -4441,7 +4441,6 @@
     "name": "crypto-sectors"
   },
   "metadata": {
-    "generated_at": "2026-05-24T17:42:15Z",
     "n_active_sub_sectors": 33,
     "n_assets": 158,
     "n_classes": 4,