Route export through portable IR boundary

[pesap]

Type

HITL

Objective

Route export through a portable IR boundary instead of compiler internals.

Human decision needed

Decide the minimum portable IR shape for current export behavior.

Acceptance criteria

• First portable IR minimum shape is approved.
• arco-ir represents enough compiled structure for current LP/MPS/export behavior.
• arco-export consumes arco-ir or a transitional portable seam, not compiler internals.
• Existing export tests pass byte-for-byte unless approved.