[codex] add stock alpha and growth pullback research suites

[Pigbibi]

What changed

• add the stock_alpha research suite and result exports
• add the growth_pullback research suite, follow-up spec-lock studies, and result exports
• add the cash-buffer daily overlay research script, test, and exported comparison files

Why

These files are a self-contained research bundle. They are not runtime changes; they package the unpublished backtest work that the later IBKR paper-line decisions were based on.

Impact

• research-only; no runtime behavior or Cloud Run config changes
• no canonical runtime profile keys changed
• keeps the stock-alpha, growth-pullback, and overlay studies reviewable together because they depend on each other

Validation

• python -m py_compile research/backtest_stock_alpha_*.py research/backtest_growth_pullback*.py research/backtest_cash_buffer_branch_daily_overlay.py tests/test_stock_alpha_*.py tests/test_growth_pullback*.py tests/test_cash_buffer_branch_daily_overlay.py
• pytest tests/test_stock_alpha_suite.py tests/test_stock_alpha_v1_robustness.py tests/test_stock_alpha_v1_1_spec_lock.py tests/test_stock_alpha_v1_1b_candidate_gate.py tests/test_growth_pullback_suite.py tests/test_growth_pullback_v1_1_spec_lock.py tests/test_growth_pullback_v1_2_geometry_repair.py tests/test_growth_pullback_v1_3_spec_normalization.py tests/test_growth_pullback_v1_4_cash_buffer_refinement.py tests/test_growth_pullback_v1_5_freeze_review.py tests/test_cash_buffer_branch_daily_overlay.py