Consider hashing duplicate transition-id validation in statum-graph

[eboody]

Context

statum-graph::validate_graph(...) currently detects duplicate transition ids by storing seen ids in a Vec and calling contains(...) for each transition, which is O(n^2) in the number of transitions.

Why this is not an immediate PR blocker

• The current code is correct.
• This path is not known to be hot today.
• Switching directly to HashSet would tighten the external generic contract for MachineDoc::try_from_graph(...) because it would require T: Hash, not just T: Copy + Eq.

Follow-up decision

Decide whether to:

1. Keep the current external API and accept the linear scan because this is not a proven bottleneck, or
2. Explicitly require T: Hash for the external path and switch duplicate transition-id detection to a HashSet.

Relevant code

• statum-graph/src/lib.rs validate_graph(...)
• statum-core/src/introspection.rs MachineIntrospection::TransitionId already requires Hash for Statum-generated graphs

Acceptance criteria

• Either document why the current Vec approach is intentional, or
• Change the bound and implementation deliberately rather than as an incidental micro-optimization.