Add comprehensive test coverage (91 → 444 tests)

[sipemu]

Summary

• Add ~50 new test cases across 10 files, bringing total from 91 to 444 passing tests
• Fix bare tibble() calls in test helpers to use tibble::tibble() for reliable namespace resolution under devtools::test()
• Add create_mock_task_with_factors() and create_fitted_mock_task() test helpers

New test coverage by area:

Area	Tests added	Files
End-to-end pipeline per model	7	test_execute.R
BMPTest, PatellZTest, CalendarTimePortfolioTest	3	test_multi_event_statistics.R
plot_event_study, plot_diagnostics, plot_stocks	12	test_plotting.R (new)
Validation edge cases	4	test_validation.R
Diagnostics edge cases	3	test_diagnostics.R
Return calculation edge cases	5	test_return_calculation.R
Cross-sectional regression	5	test_cross_sectional.R
Export content validation	3	test_export.R
Tidy edge cases	3	test_export.R

Refs #3

Test plan

• devtools::test() passes: 444 tests, 0 failures, 0 warnings

🤖 Generated with Claude Code

[sipemu]

Added: 60 edge case tests

New file test_edge_cases.R covers degenerate data and boundary conditions:

• Single-event multi-event stats: CSectTTest, SignTest, GeneralizedSignTest, RankTest, BMPTest with n=1
• NA propagation: Multi-event stats with all-NA abnormal returns
• Zero variance / constant returns: MarketModel, MarketAdjustedModel, ComparisonPeriodMeanAdjustedModel with constant data
• Division by zero: Forecast error correction with sigma≈0, PatellZTest with very short estimation windows
• Return calculation edge cases: Zero prices, negative prices (NaN), SimpleReturn zero-division
• Single-day event window: Full pipeline with event_window_length=0
• Empty event window: Event date not found in data
• BHARTTest NA coalesce: Verifying NA→0 fallback behavior
• Task accessor error paths: get_ar(), get_car() before fitting
• ParameterSet null stats: Pipeline with all statistics set to NULL
• Validation edge cases: Multiple event dates, unfitted model diagnostics
• CalendarTimePortfolioTest: Constant AARs input

Also documents a known bug: MarketModel$calculate_statistics() crashes with zero-variance index returns (subscript out of bounds) because calculate_statistics() is not wrapped in safely().

Full test suite: 504 tests, 0 failures, 0 warnings.