Adopt microunit for tax-unit reconstruction (scoped, behavior-preserving; part of #113)

[MaxGhenis]

What

Adds microunit as a dependency and introduces a delegation seam in the PolicyEngine tax-unit reconstruction path. USPipeline._build_policyengine_tax_units_via_microunit calls microunit.construct_tax_units when the person frame carries microunit's raw CPS input columns (PH_SEQ, A_LINENO, A_MARITL, A_SPOUSE, PEPAR1, PEPAR2, A_EXPRRP), and returns None otherwise.

Part of #113. The us-data side of the same adoption is PolicyEngine/policyengine-us-data#1157.

Scope — what this does and does NOT do

A scoped, behavior-preserving first step, not the full ~1,500-line replacement #113 describes.

• ✅ Adds the dependency (pinned pre-PyPI: microunit @ git+…@d3eccbb) + lockfile.
• ✅ Adds the delegation method and wires it into _build_policyengine_tax_units_from_role_flags (tried first; falls through to the legacy reconstruction on None).
• ✅ Preserves the authoritative-ID path (Restore eCPS export and entity ID parity #112) untouched — it is never routed through microunit.
• ❌ Does not delete the legacy role-flag reconstruction; it stays as the fallback.
• ❌ Does not change any output on today's data. microplex's reconstruction-stage frame collapses relationship_to_head into a 0/1/2/3 coding and drops the spouse/parent pointer columns, so the column guard is never satisfied in production → the delegation always returns None today. It is inert until a follow-up threads CPS columns through to entity construction.

⚠️ Entity-convergence caveat (do not skip — see #113)

microunit is eCPS's tax-unit construction engine. Activating this delegation (in a later PR, once CPS columns are threaded through) makes microplex's tax units converge toward eCPS's. Any loss movement from that must be read as an entity-convergence effect — not microplex improving — and isolated with a matched-N, symmetric-refit, holdout before/after comparison on the same target surface. This PR deliberately keeps the delegation inert so it can land without touching the live benchmark.

Tests

• tests/pipelines/test_us_microunit_delegation.py — 4 passing (exercises the real delegation path with a CPS-shaped fixture; microunit installed).
• ruff check clean on changed files.

Reviewer notes

• The crux is the input-schema mapping in _build_policyengine_tax_units_via_microunit: microunit keys on (PH_SEQ, A_LINENO); per-person TAX_ID/role and per-unit filing_status are mapped back onto microplex's person_id/household_id with start_tax_unit_id offsetting. Please scrutinize that mapping and the offsetting.
• Draft on purpose. Per Adopt microunit for tax-unit construction (guard against regression-to-eCPS) #113, MP adoption is sequenced after the us-data adoption (#1157) lands and is verified in production. Do not merge ahead of that.

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