Create arco-targets crate and move compile output types

[pesap]

Type

AFK

Objective

Create the solver-facing target seam so compile output types are no longer owned by authoring or compiler internals.

Acceptance criteria

• New arco-targets crate exists in the workspace.
• Solver-facing lowered types currently shared from compiler are moved or re-exported through arco-targets.
• Existing compile/export/solver tests pass.
• No solver adapter depends on authoring internals for target types.