List swap routes

Returns every share class pair you have access to that supports a swap.

• Both share classes of a returned pair are ones you can access, and the pair is one Spiko can currently convert. Use this to decide what to present: a pair absent from the list is a swap you should not offer.
• Pass fromShareClassId to get only the routes leaving one share class.
• A pair listed here is one POST /v0/swap-orders/ accepts. The converse does not hold: that endpoint gates on convertibility and on the target account holding an active line, not on your access to the share classes, which is gated when the line is added. A pair absent here only for access reasons may therefore still be accepted.
• Whether a pair is swappable depends only on the two share classes, so this response is safe to cache per distributor. It says nothing about a particular investor: balance, an active account line on the destination account, and the investor's own fund access are checked when the order is placed.