We need restart support in SIPNET so that segmented runs can stop and resume without hidden-state drift.

Scope (MVP):

• Add restart input/output options to SIPNET runtime.
• Write a versioned restart checkpoint (SIPNET_RESTART 1.0, text format) containing all runtime state needed for deterministic resume.
• On restart, run normal setup, restore checkpointed state, and enforce strict restart compatibility checks.
• Preserve cumulative tracker continuity across segments.
• Keep output stitching in orchestration (no append/stitch logic in SIPNET).

Constraints:

• Resumed climate input must start at a timestamp (year, day, time) strictly after the checkpoint boundary timestamp.
• Do not allow replay/modification of already-processed events (deterministic cursor/history checks).
• Maintain backward behavior when restart options are not used.

Validation:

• Add hop-style tests comparing continuous vs segmented runs.
• Include fail-fast tests for restart boundary/climate ordering mismatch and invalid/corrupt restart inputs.
• Confirm non-restart behavior is unchanged.

Out of scope:

• Workflow/orchestration redesign.
• Mid-run parameter hot-swapping.
• Output merge/stitch features.