M2-L01: docs(protocol): clarify session-key authorization scope and metadata extensibility

[philanton]

Summary

Audit finding M2-L01 (Low) flagged that channel session keys are authorized today only by asset and expiration, with no per-action, channel, recipient, or spending-limit enforcement. We are acknowledging the finding without changing runtime behaviour for now, and tightening the protocol documentation so the actual scope is explicit and the metadata extension model is unambiguous.

Changes

• docs/protocol/cryptography.md
  • Replaces the loose "scope, expiration and possible other data" phrasing with a reference to the canonically encoded authorization metadata structure.
  • Adds an Authorization Metadata subsection that:
    • Lists version and expires_at as the protocol-mandated minimum.
    • Explains that version is a replay / revocation guard — issuing a higher-version session-key state supersedes (revokes) prior ones — not a metadata-layout version.
    • States that implementations MAY extend the metadata with additional restrictions (authorized assets, allowed transitions, channels, recipients, spending limits, …), and that any dimension an implementation does not encode and enforce is not narrowed: a session key can sign any otherwise-valid state along that dimension up to the participant's full balance until expiry.
    • Documents the rationale for binding only the keccak256 of the metadata into the authorization signature: implementations can extend the structure off-chain without on-chain validator changes and without invalidating session keys issued under earlier layouts.
• docs/protocol/terminology.md
  • Removes the misleading "specific types of state updates" wording from the Session Key entry, links it to the cryptography section, and notes that a participant delegating a session key SHOULD assume it may exercise any authority not narrowed by the scope it issued.

assets is intentionally not promoted to the protocol minimum — it is an existing implementation-level extension and remains documented as such by omission from the minimum scope and inclusion in the example list of optional restrictions.

Test plan

• Docs-only change; no code paths affected.
• Visual review of the rendered Markdown for the two modified files.

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