canvas: define a versioned persistent graph document

[guicybercode]

Context

An infinite canvas should not make terminal components or React layout state the source of truth. The graph must be durable, forward-compatible, and independent from rendering technology.

Goal

Define a CanvasDocument domain model for nodes, edges, groups, layout, viewport, and task bindings.

Scope

• Define stable node/edge/group IDs and document revision.
• Model Terminal, Note, Task, Agent Team, File Tree, and future Portal node kinds.
• Separate node presentation data from referenced domain entities.
• Define typed connection purposes such as message, context, ownership, and visual-only.
• Persist coordinates, dimensions, collapsed state, z-order, groups, and viewport.
• Add optimistic concurrency and schema migrations.
• Preserve unknown node kinds for forward compatibility where safe.

Acceptance criteria

• Deleting a visual node does not delete its task, session, worktree, or note unless separately confirmed.
• A terminal node references a session ID; it never owns PTY state or replay bytes.
• Graph cycles are allowed for visual/message links but validated for orchestration semantics.
• Stale document writes cannot overwrite a newer layout.
• Import/export has explicit size and node-count limits.
• Documents survive daemon and UI restart.
• Serialization, migration, corruption, and referential-integrity tests are present.
• The format is documented before UI implementation.

Out of scope

Canvas rendering and real-time multi-user editing.