Restore strike_benefits from the OI_VAL decomposition (#591 — ECPS parity)

[MaxGhenis]

The pool's alimony operator recognized OI_OFF code 12, stripped it from miscellaneous_income, and discarded the amount; the archived ECPS carried it (cps["strike_benefits"] = (OI_OFF == 12) * OI_VAL). This restores the category with the existing operator as sole owner of the OI_VAL split — no new operator, pre-clone phase, float64, exact archived semantics.

• Conservation test: alimony + strike benefits + miscellaneous income == OI_VAL rowwise.
• Transfer plan: joins the CPS-carried ACS targets (114 → 115; hash updated deliberately); dtype, ownership, and consumer guards all pass (13 live engine consumers incl. market_income).
• Materiality note: the 2024 ASEC checkpoint contains zero code-12 rows, so this vintage's artifacts are numerically unchanged — the fix is structural (the category is no longer silently discarded) and covered synthetically.

Verification: focused 197, affected modules 709, #583 guard 495, full workspace 4,936 passed / 60 skipped. Ruff + format + git diff --check clean.

Part of populace#591 (classification receipts in the issue thread).

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