Triage: remove (or justify) provider-initiated checkpoints

[bkontur]

Summary

Provider-initiated checkpoints were added to the design (#7) and implemented / documented (#92), but on review the design does not hold up and may not be needed at all. This issue tracks the decision to either justify & canonicalize it or remove it entirely.

As a first step, the design has been pulled out of the review-gated canonical docs in #305 (recoverable from git) so it no longer reads as ratified while under question.

Why it's questioned

Claim in the draft	Reality
"Mobile clients can't put a checkpoint on chain"	The client uploads the data — it is the sole holder — so it can submit the checkpoint on-chain in the same session. No always-on infrastructure needed.
"There is no protection without an on-chain checkpoint"	False. A client that retains the providers' signed commitments can enforce / challenge that off-chain-signed state directly.
Provider-initiated coordination is a new capability	It overlaps what replica nodes already do — autonomous sync + challengeability.
The "Why" section justifies the feature	It was never reviewed and does not hold up under scrutiny.

Decision needed

• Option A — Approve & canonicalize. The design is reviewed and accepted; move it back into docs/design/ (review-gated) with a corrected rationale, and keep the implementation.
• Option B — Remove entirely. Delete the related code (checkpoint_coordinator.rs, the pallet extrinsics / storage / events) and the design draft, as redundant with replica nodes + client-held signatures.

What #305 already changed

Removed the entire provider-initiated checkpoint surface from docs/design/scalable-web3-storage-implementation.md:

Layer	Removed
Config	DefaultCheckpointInterval, DefaultCheckpointGrace, CheckpointReward, CheckpointMissPenalty
Storage	CheckpointConfigs / CheckpointWindowConfig, LastCheckpointWindow, CheckpointRewards, CheckpointPool
Events	ProviderCheckpointSubmitted, CheckpointConfigUpdated, CheckpointMissPenalized, CheckpointRewardClaimed, CheckpointPoolFunded
Signed payload	CheckpointProposal (struct + signing reference)
Extrinsics	provider_checkpoint, configure_checkpoint_window, report_missed_checkpoint, claim_checkpoint_rewards, fund_checkpoint_pool
Off-chain HTTP	/checkpoint/sign, /checkpoint/duty, /checkpoint/trigger

Not touched: client-initiated checkpoint / extend_checkpoint / challenge_checkpoint remain in the canonical docs — this issue is only about the provider-initiated variant.

The design content now lives in docs/drafts/provider-initiated-checkpoints.md pending this decision.

References

• docs: CODEOWNERS review gate for docs/design + move drafts out #305 — removal + docs reorg
• feat: add checkpoint protocol, challenge responder, and replica sync coordinator + CI #7 — design introduced
• chore: update web3 storage implementation docs #92 — implementation documented