Support NFTs as NFT minter and assigned-owner identities

[Rigidity]

Motivation

The action system currently treats a non-XCH NFT mint parent and a non-empty NFT current_owner as a DID:

• MintNftAction resolves parent_id only from Spends::dids.
• TransferNftById resolves its assigned identity only from Spends::dids and builds the concurrent assignment announcements from that DID spend.

XCHandles NFTs are useful on-chain identities in their own right. Wallets should be able to use an owned NFT launcher as the minter of a new NFT or as another NFT's assigned owner without reimplementing action-system spend composition.

Scope

• Generalize the identity input used by NFT mint and transfer actions so it can resolve a spendable DID or NFT.
• Allow an NFT spend to:
  • parent a newly launched NFT, making the parent NFT launcher the assigned minter;
  • emit/assert the required concurrent announcements when its launcher is assigned as another NFT's current_owner.
• Preserve the existing DID-oriented helpers and behavior for source compatibility.
• Add explicit generic/NFT helpers or typed identity selection so callers do not rely on ambiguous lookup order.
• Reject unsupported singleton kinds and attempts to use missing or already-spent identities with a clear driver error.

Acceptance criteria

• An NFT can mint a child NFT through the high-level action system.
• An NFT can be assigned as another NFT's current_owner through the high-level action system.
• Existing DID mint/assignment APIs and tests remain compatible.
• Simulator tests cover ordinary NFTs and XCHandles-compatible NFT metadata.
• Tests cover chained NFT ownership, missing identity assets, and concurrent spend announcements.
• Public API docs explain that the stored identity is a singleton launcher ID, despite the historical DID-oriented naming.

Downstream

This unblocks generic NFT identity support in Sage while allowing Sage's initial UI to limit choices to active, verified XCHandles NFTs.