chore(hmm): verify remaining type-check, format, and smoke-run ACs from #2 and #3

[Pepeye]

Context

Follow-up to issues #2 and #3. The following acceptance criteria were not verified during the initial implementation pass and remain open.

Unmet Acceptance Criteria

From #2 (_rebuild_regime_rank_cache refactor)

• uv run ty check shared/core/hmm tests/hmm reports no in-scope errors

From #3 (HMM walk-forward backtest runner)

• uv run ruff format --check scripts tests/hmm exits 0
• uv run ty check scripts tests/hmm reports no in-scope errors
• Live smoke run: uv run python scripts/hmm_backtest.py --ticker SPY --start 2020-01-01 --end 2024-12-31 exits 0 and prints Final equity / Total return / Sharpe / Max drawdown / Regime distribution

Acceptance Criteria

• All four items above pass cleanly
• No regressions: uv run python -m pytest tests/hmm -q still passes after any fixes