standards: add DRC721 primitive and reference

[HDauven]

Part 5 of the Dusk standards stack.

Adds the DRC721 standards layer and Forge reference contract:

• DRC721 primitive
• ownership, approval, operator, transfer, burn, and enumeration support
• royalty registry support
• signed NFT approval/operator authorization
• DRC721 collection Forge reference contract
• focused VM coverage for the DRC721 reference contract signed admin/approval paths

Tracking:

• Standard tracker: Track DRC721 non-fungible token standard #46

Follow-ups not completed by this PR:

• Document DRC721 standard semantics and Dusk principal model #47 Document DRC721 standard semantics and Dusk principal model
• Add DRC721 event and data-driver compatibility fixtures #48 Add DRC721 event and data-driver compatibility fixtures
• Add DRC721 wallet/client signed approval examples #49 Add DRC721 wallet/client signed approval examples

Review focus:

• token ownership and approval invariants
• operator burn/transfer behavior
• pause semantics
• royalty validation and event coverage
• signed approval nonce/payload binding
• Forge reference contract VM behavior

Validation:

• make standards-ci (includes DRC721 primitive tests, DRC721 reference Wasm build, and non-ignored DRC721 VM coverage)
• make standards-data-drivers
• cargo +nightly-2026-02-27 test --test drc721_reference_vm -- --nocapture from standards/dusk-contract-standards