Add a hint to the send action

[Rigidity]

Note

Low Risk
Small, localized change to memo/hint selection for send transactions; no changes to spend construction beyond adding a memo entry.

Overview
Adds an automatic Hint::P2PuzzleHash(receiver_puzzle_hash) when building Send actions for non-XCH assets, so CAT sends include a receiver puzzle-hash memo hint even without clawback.

This only affects memo calculation for sends; clawback handling and puzzle-hash selection remain unchanged.

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