[#552] Enforce auditable commitment payment-token allowlists

[mikewheeleer]

Summary

• Keep commitment marketplace payment tokens behind the existing admin-controlled allowlist.
• Make additions and removals auditable with authenticated TokenAdd and TokenRem events.
• Preserve idempotent behavior for duplicate additions and removals of absent tokens.
• Document that removing a token leaves existing commitment data intact while preventing new settlement use until re-allowlisted.

Acceptance criteria

• Unsupported tokens are rejected before funds move by the existing allowlist checks on listing, offers, auctions, and settlement.
• Allowlist changes require the current admin and emit auditable events.
• Removing a token does not rewrite existing listings or commitments.
• Tests cover duplicate/absent updates, allowlist state, and removed-token settlement behavior.

Validation

• git diff --check
• cargo test -p commitment-marketplace (blocked by a pre-existing syntax error in contracts/commitment_nft/src/tests.rs, outside this PR; the compiler reports mismatched delimiters before compiling this package.)

Security and compatibility

The allowlist remains fail-closed and is checked before external token transfers. Events are emitted only after the corresponding storage mutation succeeds. This is backward-compatible for already approved tokens; rollback is a simple revert of the event/documentation changes.

Closes #552