Stage 1 rerun reuse and checkpoint adapter boundary

[anth-volk]

Context

Stage 1 PR 4 introduces an explicit checkpoint adapter and semantic rerun/reuse planner while preserving the current physical checkpoint layout.

Scope

• Add CheckpointStore, checkpoint decisions, and prior-compatible reuse counters.
• Add Stage1RerunPlanner, Stage1ReuseDecision, and Stage 1 identity material.
• Route existing Modal checkpoint helpers through the store while preserving /checkpoints/{branch}/{sha} behavior.
• Attach reuse/checkpoint decisions to substep results, status events, and Stage 1 contract metadata.

Validation

Targeted unit, docs, quality, and lint checks will be reported on the linked draft PR.