Add conservative deterministic fast paths for unambiguous commands #45

Goal

Avoid an AI call when Lucid can safely execute a command from exact wording and current UI state.

Depends on

• Measure AI latency, token use, repairs, and edit isolation #34 for measurement.
• Build a lightweight, rebuildable document context index #42 so fast paths can resolve stable targets safely.

Scope

Start with a very small allowlist, such as selected-block delete/move actions and creation of an explicitly named empty block where the target is unambiguous. Matching must be exact and state-aware.

Acceptance criteria

• Ambiguous wording always falls back to the model or clarification.
• Destructive operations retain confirmation rules.
• Fast-path changes still produce BlockPatch, history, and one-step undo.
• Unit/E2E tests include near-miss phrases that must not be intercepted.
• Metrics distinguish deterministic execution from model execution.
• Do not introduce a routing model.