chore: clean package static-analysis findings

[shawcharles]

Closes #606.

Summary

• declare the ResolvedSurveyDesign forward reference in bootstrap_utils.py under TYPE_CHECKING
• remove no-placeholder f-strings from power.py
• remove the stale invalid # noqa: WPS433 marker from spillover.py while preserving the deferred-import note
• remove audited unused locals where the computed value has no downstream use
• document intentionally omitted symmetric DRDID grid entries in staggered.py
• add an Unreleased changelog entry

Scope

This intentionally does not touch:

• public API re-exports / F401
• TROP import ordering / E402
• broad import sorting / I001
• honest_did.py mathematical l names / E741
• CI gates

Validation

• python -m ruff check diff_diff --select F821,F541,F841,W --output-format=concise
• python3.9 -m py_compile diff_diff/bootstrap_utils.py diff_diff/power.py diff_diff/spillover.py diff_diff/chaisemartin_dhaultfoeuille.py diff_diff/honest_did.py diff_diff/imputation.py diff_diff/prep_dgp.py diff_diff/staggered.py
• git diff --check
• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_bootstrap_utils.py tests/test_bootstrap_chunking.py -q (74 passed)
• PYTHONPATH=. DIFF_DIFF_BACKEND=python pytest tests/test_power.py tests/test_methodology_power.py tests/test_spillover.py tests/test_honest_did.py tests/test_methodology_honest_did.py tests/test_imputation.py tests/test_methodology_imputation.py tests/test_prep_dgp_reversible.py tests/test_staggered_rc.py tests/test_staggered.py tests/test_chaisemartin_dhaultfoeuille.py tests/test_dcdh_heterogeneity_cell_period_coverage.py -q (1390 passed, 103 deselected)