[#551] Guard commitment NFT storage migrations

[Baskarayelu]

Summary

• record the current schema version for fresh commitment NFT deployments
• reject unsupported and mismatched source versions before migration writes
• require the legacy admin, counter, and token-index schema before proceeding
• preserve the v1 instance-storage token index when moving to persistent storage
• initialize only the missing reentrancy guard and write the v2 marker last
• add representative old-layout fixtures covering migration, authorization, idempotency, and partial state
• document storage keys, version meanings, write ordering, and operational recovery

Issue coverage

• Unsupported storage versions fail before mutation
• Existing commitments and balances retain their meaning after migration
• Migration is authorized and idempotent
• Tests cover old fixtures, partial state, repeated migration, and unsupported version data
• Existing package checks remain green
• Design, tradeoffs, test evidence, and limitations are documented

Validation

• cargo +1.88.0 test -p commitment_nft — 20 passed
• rustfmt --edition 2021 --config skip_children=true --check contracts/commitment_nft/src/lib.rs
• git diff --check
• Diff: 508 additions, 7 deletions

Note: the repository’s legacy feature-gated src/tests.rs contains a pre-existing parse error that prevents recursive cargo fmt; the changed files were checked without traversing that unrelated module.

Closes #551