Implement `fn snapshot` / `fn restore` over the declared StateConfig contract

[hyperpolymath]

Summary

crates/wharf-core/src/config.rs declares the snapshot-system
config surface:

• StateConfig.snapshots_to_keep (default 10)
• StateConfig.snapshot_dir (default .wharf/snapshots)

and crates/wharf-core/src/mooring_client.rs references
commit_resp.snapshot_id as the reference key returned from a
successful mooring commit.

But there is no actual snapshot implementation yet — a workspace-wide
search for fn snapshot, fn restore, fn recover, and
fn create_snapshot returns 0 hits across crates/, bin/, and
adapters/. The contract is documented; the runtime is nascent.

Surfacing

veridical-simulation-core's
wharf-veridicality-probe (Phase 4c of the Veridical Simulation
program-of-work, 2026-04-27) ran a reference snapshot ledger
against the declared contract — 10 rounds, snapshot-round-trip-T-N
= 1.0000, retention-policy-honoured = 1.0000 — and then probed the
project-wharf source for the real implementation:

implementation-presence: 0.0000
  fn snapshot         = 0
  fn restore          = 0
  fn recover          = 0
  fn create_snapshot  = 0

Per-shape geometric mean against the declared config contract is
0.9647; the impl gap is the standalone finding.

Acceptance criteria

A minimum viable snapshot subsystem under crates/wharf-core/src/
that:

• Materialises each snapshot under <snapshot_dir>/<snapshot_id>/
  with at least a payload.bin plus a manifest.json carrying the
  ledger ID, round/sequence number, and SHA-256 of the payload.
• Provides fn create_snapshot(state, id) -> Result<Snapshot>
  and fn restore(id) -> Result<State> with byte-exact round-trip:
  restore(id) returns the bytes that create_snapshot wrote.
• Honours snapshots_to_keep retention on every write — never
  exceeds the budget on disk.
• Exposes the snapshot_id key the mooring CommitResponse already
  surfaces, so the existing client wiring works unchanged.

The reference ledger inside wharf-veridicality-probe
(rust-core/wharf-veridicality-probe/src/main.rs, the
Snapshot / write_snapshot / read_snapshot / enforce_retention
functions) is a working blueprint that respects the same contract
and can be lifted into wharf-core more or less directly.

Reproducer

cargo run -p wharf-veridicality-probe -- \
  --wharf-config-rs /var/mnt/eclipse/repos/project-wharf/crates/wharf-core/src/config.rs \
  --wharf-root      /var/mnt/eclipse/repos/project-wharf \
  --snapshot-rounds 10 \
  --api 'http://[::1]:8080' \
  --out /tmp/wharf-phase4c.a2ml

After the impl lands, the same probe should report
implementation-presence: 1.0000 and the existing 1.0 round-trip /
retention scores should hold.

🤖 Generated with Claude Code