M1: Domain-separated digest context for RenderGraph canonical bytes

[flyingrobots]

Pulse context: domain separation in Merkle-style graph hashing.

Goal

• Ensure echo-graph RenderGraph digest is domain-separated (derive-key context) and versioned so it cannot be confused with other digests.

Scope

• Add/confirm an explicit encoding version tag in the canonical byte stream.
• Compute digest using a derive-key context (e.g. ECHO/echo-graph/render_graph_digest/v1).

Acceptance criteria

• Unit test: same bytes hashed with a different domain yields different digest.
• Any existing viewer/session code that relies on this digest updated accordingly.

[flyingrobots]

All requirements shipped in PR #265 (merged 2026-02-13). Milestone docs finalized in PR #266 (merged). Closing.