fix(prices): skip Yahoo-only markets (MME/MFS) in the Norgate producer

[mspinola]

Problem

Running cotdata-update --metadata (Windows producer) errored on two symbols:

ERROR: Norgate Data: security_name: &MME_CCB not found
ERROR: Norgate Data: security_name: &MFS_CCB not found
... (every spec field, for both symbols)
Successfully wrote metadata for 49 symbols -> store

Root cause: MME and MFS are MSCI international indices Norgate doesn't carry — they're priced off ETF proxies (EEM/EFA) via the yfinance provider. But registry.py defaulted every symbol's Norgate ticker to &<internal>, so they silently became &MME/&MFS. The Norgate producer iterated all symbols and tried to fetch nonexistent &MME_CCB/&MFS_CCB. --prices failed those cleanly per-symbol, but --metadata catches each field error to None and silently wrote two all-null spec rows to contract_specs — the real harm.

Fix

Mirrors how the yfinance provider already filters by s.yahoo:

• Mark MME/MFS norgate: null in registry.yaml (Symbol.norgate is now Optional[str]).
• Add _norgate_covered() in the Norgate provider, applied in both update() (prices) and update_metadata(), to skip symbols with no Norgate coverage — with a one-line note instead of erroring.

Now the producer prints skipping 2 symbol(s) with no Norgate coverage (priced elsewhere): MME, MFS and processes only the 47 real Norgate markets.

Store cleanup

The two all-null rows already written to the store were pruned directly via the store API (49 → 47 rows, atomic write + manifest update). Not part of this diff — it's a data change, outside version control — but already applied, so no producer re-run is required to correct the data.

Tests

• test_registry.py: MME/MFS norgate is None; ES still defaults to &ES.
• test_norgate_provider.py: _norgate_covered unit filter + full-run update_metadata store round-trip proving MME is never fetched and no null row is written.
• Full suite: 70 passed.

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