[codex] Migrate global ETF rotation to feature snapshot

[Pigbibi]

Summary

• Migrate global_etf_rotation runtime input from market history to feature_snapshot.
• Add Global ETF snapshot runtime adapter contract metadata and diagnostics.
• Update tests/docs for Global ETF snapshot-backed runtime and retired Russell defensive references.

Validation

• PYTHONPATH=src .venv/bin/python -m unittest tests.test_entrypoints tests.test_catalog tests.test_global_etf_rotation
• git diff --check

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Gate rebalance on the snapshot date

For artifact-backed runs where the feature snapshot was produced on the quarter-end but ctx.as_of is the platform run date (for example, running on 2026-04-01 with a 2026-03-31 snapshot), passing ctx.as_of makes _snapshot_rebalance_day evaluate April 1 and return daily_check, so no quarterly weights are emitted. This should use the snapshot's as_of date, or an explicit post-snapshot execution window, rather than the runtime date.

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