Feat: Add enhancements, Update enforcement mechanisms and Add MPP

[emg110]

This PR provides:

• General edits and enhancements
• Addition and completion of enforcement mechanisms
• Adding MPP to supported use cases
• Adding extension architecture support to the core AC2 spec.

Note: After this PR merger, upcoming extensions are ready to be pushed:

• Preauthorized autonomous payments extension.
• AC2 Discovery extension (complying to Bazaar and MPP discovery specs).
• AC2 A2A extension and Agent-to-agent communication and control .
• AC2 AP2 extension to provide AP2 Verifiable Credential signing using AC2.

[Copilot]

Pull request overview

This PR updates the AC2 core specification to clarify core invariants, add an explicit extensions model (including MPP use cases), and introduces a new “Pre-Authorized Operations” extension for bounded autonomous payments/asset transfers.

Changes:

• Refines AC2 core language around key custody, signature request semantics, and adds extension architecture + capability namespacing guidance.
• Adds MPP (charge/session) use cases and related references.
• Introduces ac2-ext-pre-authorized defining a Pre-Authorized pattern and messages for top-ups, grants, and spend receipts.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 7 comments.

File	Description
ac2.md	Updates core AC2 protocol narrative, data model examples, and adds extension architecture + additional message examples.
ac2-ext-pre-authorized.md	New extension spec defining pre-authorized autonomous payment/transfer flows and associated DIDComm messages.

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