recursion v2: minimize copied OpenVM system interfaces

[kunxian-xia]

Parent: #1266

Audit ceno_recursion_v2/src/system and minimize copied OpenVM recursion system interfaces.

Goal

Keep Ceno recursion v2 aligned with OpenVM develop-v2.1.0-rv64 by re-exporting upstream traits/structs whenever their signatures and semantics work unchanged, and keeping local definitions only where Ceno needs different associated types, local buses, or private upstream data.

Expected changes

• Re-export upstream system traits/structs where possible instead of copying them locally.
• Keep local adapters only for Ceno-specific ownership, such as RecursionVk, RecursionProof, local BusInventory, and adapted preflight records.
• Clearly document any remaining local fork/adaptation so future OpenVM upgrades can distinguish intentional divergence from drift.
• Avoid silent duplicate traits with the same name/meaning as upstream traits.

Acceptance criteria

• ceno_recursion_v2/src/system has the smallest practical local adapter surface.
• Traits whose signatures match upstream are imported/re-exported from recursion_circuit::system.
• Local definitions that remain have explicit comments explaining why upstream cannot be used directly.
• Existing recursion v2 tests still compile and pass after the cleanup.