Revert "Drop the four CBOE implied-vol indices from the registry"

[mspinola]

Reverts the merge of #19, and adds the note whose absence caused it.

Why

#19 removed VIX, VXN, OVX and GVZ on the grounds that nothing kept them. Something does: npf/trading_riot maps each to a realized counterpart (VIX→ES, VXN→NQ, OVX→CL, GVZ→GC) for a variance-risk-premium study, and its data.py documents fetching them with --bars --symbols VIX.

#19's own commit message recorded that dependency — as a consequence to accept. It is a reason not to remove them.

Its other premise expired

"Store side is already done on the producer: the four parquets and their manifest entries are gone, leaving 11 equities symbols, which is what this registry now lists."

True when written; reverted on the producer shortly afterwards. That left registry and store disagreeing — registry 11, store 15 — and since nothing prunes unregistered symbols, the four would have sat in the store as orphans no run ever refreshed again, mirrored to both replicas in that state by the next sync.

After this change they agree at 15, verified against the producer's manifest:

registry: 15   store: 15
registered but not in store: none
in store but unregistered  : none

The note

These four have now been removed twice, both times because a grep of this repo comes back clean while the note recording their retirement lived somewhere else. So the reason they are registered is written next to them in registry.yaml, not in a document that can drift out of step with the file that actually runs:

Absence of a local reference is not evidence of absence of a dependency.

It also says what to do if trading_riot really is retired: delete the four and say so there, in the same commit, rather than leaving the next reader to rediscover the dependency from a failed fetch.

Verification

ruff check src tests clean. pytest tests/ -q -m "not network" — 205 passed, 12 deselected. The tests are insensitive to this either way since #20; they ask the registry for a symbol rather than naming one.

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