Create eCPS replacement microsimulation benchmark suite for mp-300k

[MaxGhenis]

Parent plan: PolicyEngine/microplex-us#16

Goal: codify the microsimulation comparisons required before Microplex can replace eCPS.

Deliverables:

• Add a frozen benchmark manifest covering SSI asset limits, CTC/EITC-style tax reforms, capital-gains indexing, and the Tara Watson SSI asset-limit benchmark.
• Pin reform definitions, periods, expected output fields, candidate artifacts, and baseline artifacts.
• Report aggregate fiscal impact, household net income, winners/losers, poverty/SPM where applicable, and component deltas.
• Enforce PolicyEngine MicroSeries operations throughout; no manual weight math.

Exit criterion: every mp-300k candidate can produce a comparable benchmark report against pinned eCPS.

[MaxGhenis]

Umbrella discussion: PolicyEngine/microplex-us#16