Remove SSTB/QBI compat shim, require policyengine-us 1.637.0

[MaxGhenis]

Summary

Collapse the two-sided SSTB/QBI implementation that landed across #701 (data) and policyengine-us#7944 (model). With policyengine-us 1.637.0 shipping the SSTB inputs and formulas natively, the compat shim in policyengine_us_data/utils/policyengine.py no longer has a job to do.

• Bumps the policyengine-us floor from >=1.572.5 to >=1.637.0 and regenerates uv.lock.
• Deletes the 357-line ensure_policyengine_us_compat_variables function that monkey-patched CountryTaxBenefitSystem.__init__ to backfill sstb_self_employment_income, sstb_w2_wages_from_qualified_business, sstb_unadjusted_basis_qualified_property, sstb_self_employment_income_would_be_qualified, sstb_qualified_business_income, total_self_employment_income, and the SSTB-aware QBI/QBID formulas.
• Removes the three import-time and validate_database call sites that triggered the shim.
• Drops the three compat-specific unit tests in tests/unit/test_package_imports.py — they exercised the shim's QBID arithmetic, which is now covered by policyengine-us's own test suite.
• Renames test_validate_database_accepts_compat_variables → test_validate_database_accepts_total_self_employment_income since the variable is no longer compat.

Net diff: +14 / −578 lines across 7 files (plus the lockfile).

Why

The original shim was a pragmatic bridge while policyengine-us and policyengine-us-data shipped the split on independent release cadences. Keeping it indefinitely has two costs:

• Policy formula logic (QBI split, QBID caps) living in a data package duplicates what policyengine-us owns and risks drift on future tweaks.
• A global CountryTaxBenefitSystem.__init__ monkey-patch is a side effect that's hard to reason about for downstream consumers.

The shim was self-disabling (needs_sstb_qbi_compat = "sstb_qualified_business_income" not in tbs.variables), so removing it is purely a dead-code cleanup for any consumer on policyengine-us 1.637.0+.

Test plan

• ruff format --check . — clean
• uv lock — resolves cleanly with 1.637.0
• PR CI: unit tests, smoke test, changelog fragment, docs build
• PR CI: integration tests (triggered by the policyengine-us-data/ changes)

Note: changelog fragment is named +slim-sstb-compat.changed.md as a placeholder; will rename to <PR_NUMBER>.changed.md after this PR lands its number.

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