Status: done

# 07 — FRAUD: On-demand commitment + hop bisection blame

## What to build

On audit selection, require nodes to supply TOPLOC-style fingerprints of **output boundary activations** per hop (on-demand, brief retention). On verify failure, referee identifies the **first divergent hop** — not always the last text node.

**Code refs:**

- `packages/validator/meshnet_validator/__init__.py` — `_slash_route`, `_final_text_node` bug (~102–140) — blames `max(shard_end)` only
- `packages/tracker/meshnet_tracker/server.py` — route hop construction (~1774–1783) — cut-points for bisection
- Research: `.scratch/alpha-hardening/research-verifiable-inference.md` §1.2, §8 layer 3 (Verde **pattern**, not on-chain game)

## Test-first

1. Red: two-hop route, corrupt hop-0 activations — `_final_text_node` blames hop-1 — test must fail.
2. Green: bisection selects hop-0; forfeit targets hop-0 wallet.
3. On-demand: commitment requested only when audit flag set on proxied request.

## Acceptance criteria

- [x] Audit requests carry tracker RNG/VRF flag indistinguishable from normal traffic (research §6) — the existing post-hoc `sample_rate` RNG gate in `ValidatorProcess.validate_once` already decides audit selection after the original proxied request completed, so the request the client/nodes saw is unaffected either way; locked in by `test_hop_commitments_are_not_requested_unless_the_event_is_audit_selected`
- [x] Nodes retain recent boundary activations for on-demand commit window (configurable TTL) — `ToplocAuditConfig.commitment_ttl_seconds`; expired commitments fall back to the text-only path (`test_expired_commitment_window_falls_back_to_text_only_audit`)
- [x] Validator/tracker compares fingerprints at each hop cut-point; first mismatch = culprit — `_hop_commitments_from_event` + `_first_divergent_hop` in `packages/validator/meshnet_validator/__init__.py`
- [x] `_final_text_node` removed or limited to text-only fallback — only called from the plain-text divergence branch of `_validate_event` now
- [x] Integration test: multi-hop pipeline, fault injected at known hop — `tests/test_hop_bisection.py`

## ADR links

- [ADR-0018](../../docs/adr/0018-fraud-detection-verification-and-reputation.md) §3–4

## Blocked by

- `06-fraud-toploc-integration_completed.md`