Fail closed on incomplete HK price sources

[Pigbibi]

Summary

• Make synthetic price data available only when price_source=synthetic and research_only=true are both explicit.
• Mark every synthetic run promotion_eligible=false.
• Park incomplete or failed provider downloads without passing partial data to the backtest or silently substituting synthetic data.
• Preserve this as research-only behavior; no provider/live execution or promotion artifact was invoked.

Validation

• PYTHONPATH=src python3 -m pytest -q — 236 passed.
• python3 -m ruff check src/hk_equity_snapshot_pipelines/snapshot_proxy_backtest.py tests/test_snapshot_proxy_backtest.py — passed.
• python3 -m compileall -q src — passed.
• git diff --check — passed.
• Sensitive-field diff scan — no matches.

Scope

Only the proxy backtest implementation and its tests are changed.