Narrow arco-solver to platform orchestration

[pesap]

Type

AFK

Objective

Narrow arco-solver to solver platform orchestration and move shared adapter contracts out.

Acceptance criteria

• Registry, preflight, selection, and solve orchestration remain in arco-solver.
• Shared adapter types move to arco-contracts.
• Solver adapters depend on contracts/targets/runtime, not arco-solver internals.
• Dependency graph proves adapters are siblings.