Add healthcare release-over-release comparison view

[daphnehanse11]

Context

The healthcare target tab already supports scope=healthcare for ACA, Medicaid/CHIP, and Medicare targets, with healthcare-specific KPIs for the latest release or a selected release. The compare page already supports release-to-release target matching with generic filters for source, measure, geography, improved/regressed, and under/over target.

What is missing is the combined workflow: show healthcare targets over releases.

Proposed implementation

• Add a healthcare scope to /populace/compare using the same target scope as the healthcare target tab.
• Treat healthcare targets as ACA marketplace, Medicaid/CHIP, Medicare, and related PTC targets from the existing healthcare target predicate.
• Add a one-click Healthcare segmented control or preset on the compare page.
• Add healthcare summary cards for:
  • ACA marketplace
  • APTC recipients
  • Medicaid
  • Medicaid + CHIP
  • CHIP implied
  • Medicare Part B
• For each healthcare summary card, show A error → B error and the fit change.
• Preserve compare state in the URL, including scope=healthcare, a, and b.
• Add a deep link from the healthcare target tab: “Compare this release to previous”, pointing to /populace/compare?scope=healthcare&a=<previous>&b=<current>.

Acceptance criteria

• /populace/compare?scope=healthcare&a=<previous>&b=<current> loads directly into healthcare-scoped comparison.
• Compare summary totals, added/removed counts, improved/regressed counts, mover lists, and the all-target table reflect the healthcare-scoped target surface.
• Switching between All targets and Healthcare updates the URL.
• The healthcare target tab links to the previous-vs-selected release healthcare comparison when a previous release exists.
• Generic compare filters continue to work within the healthcare scope.