fix(storage): preserve the EIA-930 fuel mix in the degenerate write path

[oldhero5]

Summary

power.generation_by_fuel was silently losing ~90% of its data on every hourly ingest: _canonicalize and write_bars in src/energex/core/storage.py deduplicated on the valid_time index alone, while the EIA-930 generation-by-fuel connector emits ~10 rows per (BA, hour) distinguished only by the fuel_type column (schema POWER_GEN_BY_FUEL requires uniqueness over [instrument_id, valid_time, fuel_type]). Only one arbitrary fuel per hour survived.

Fix

• New _dedup_key() helper: the Datetime index, extended to a (Datetime, fuel_type) MultiIndex when a fuel_type column is present.
• _canonicalize and both write_bars paths (new-row check, interior-insert dedup) key on it.
• Stable sort_index: ArcticDB validate_index=True only requires a sorted index — duplicate timestamps across fuels are fine.
• Frames without fuel_type behave exactly as before; keep="last"/append-only semantics unchanged.

Tests

New tests/test_write_bars_fuel_mix.py (written first, all 6 failed on the old code): multi-fuel roundtrip, idempotent rewrite, interior sparse insert, append-after-tail, new fuel at an existing hour, canonicalize-level dedup. Full suite: 268 passed (test_write_bars_sparse.py and test_storage_roundtrip.py untouched and green).

End-to-end verification

Materialized the real eia930_generation_by_fuel Dagster asset (live EIA API → quality gate → write_bars) into a scratch lmdb Arctic store: 72 BA symbols, 33,386 rows, 15 distinct fuel types; CISO stores 9 fuels/hour, ERCO/MISO 8 (previously 1). Re-materializing the same partition changed nothing (idempotent), no duplicate (hour, fuel) pairs, all indexes sorted.

Follow-up

• Backfill: hours ingested before this fix hold one arbitrary fuel each. EIA-930 retains history, so re-materializing eia930_generation_by_fuel over the affected window recovers the full mix; re-ingest adds missing fuels without touching stored rows.
• _observed_fuel_types on feat/entity-graph needs no change — it heals automatically once the store carries the mix. The doc caveat in website/docs/entity-graph.md (that branch) has been updated separately.

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