Triage docs/drafts: review & promote shipped-feature docs (marketplace, checkpoint manager, encryption, …)

Summary

The docs reorg in #305 moved everything not-yet-canonical into docs/drafts/. But that folder now conflates two very different kinds of doc:

1. Genuinely unratified / questionable WIP — tracked separately (provider-initiated checkpoints → #306; "Capped Split" challenge-economics extension).
2. Accurate documentation of shipped features that were only swept in by the bulk move.

This issue triages group (2): review each doc and promote it to docs/design/ (review-gated) or docs/reference/ (ungated), or consciously keep it as a draft.

Verified implemented — review & promote

Checked against code; each accurately describes shipped functionality:

• marketplace.md — provider capacity + discovery/matching.
  • Code: client/src/discovery.rs (DiscoveryClient, find_matching_providers, suggest_providers, find_best_provider, MatchedProvider); pallet max_capacity + enforcement (crates/pallets/storage-provider/src/impls/providers.rs); providers_with_capacity runtime API.
  • Nit: MinStakePerByte is shown as both 1_000_000 (§Economic Model) and 1_000 (§Configuration) — reconcile.
• CHECKPOINT_PROTOCOL.md — client-side (Layer 1) checkpoint manager. This is the client-initiated orchestration (the ratified path), not the provider-initiated feature removed in #305 / #306.
  • Code: client/src/checkpoint.rs (CheckpointManager, CheckpointConfig, CommitmentCollection, CheckpointResult, ProviderHealthHistory, start_checkpoint_loop).
  • Nits: two Phase-3 items still unchecked (auto-challenge for divergent providers, metrics/monitoring); reconcile the submit_commitment(...) reference against the actual checkpoint extrinsic name.
• CLIENT_SIDE_ENCRYPTION.md — client-side encryption wire format + ciphers.
  • Code: Rust client/src/encryption.rs (EncryptionKey, XChaCha20Poly1305Cipher, v0x01); browser user-interfaces/s3-ui/src/lib/encryption.ts (AES-256-GCM, v0x02). Wire format matches the doc.

Also in docs/drafts/ — triage the same way (not yet code-verified)

• L1_design_implementation.md (file-system & S3 interfaces — implemented)
• S3_METADATA_INDEX.md
• smart-contracts.md (already flagged in docs/README.md as a reference/ candidate)
• EXECUTION_FLOWS.md

Decision needed (per doc)

For each: docs/design/ (review-gated — needs @eskimor per CODEOWNERS) if it's design-of-record, docs/reference/ (ungated) if it's how-it-works reference, or keep as draft. Fix the nits above on promotion.

Out of scope (tracked elsewhere)

• Provider-initiated checkpoints → #306
• "Capped Split" / challenge-economics two-tier gap → docs/drafts/challenge-economics-extensions.md

Refs: #305, #306

Start with the checklist in docs/drafts/ and compare the verified documents against the referenced Rust and TypeScript entry points, including client/src/discovery.rs, client/src/checkpoint.rs, client/src/encryption.rs, and user-interfaces/s3-ui/src/lib/encryption.ts. Review the listed nits and determine whether each document belongs in docs/design/, docs/reference/, or remains a draft. Done means every listed document has a recorded disposition and required fixes are addressed.