feat(ops): E2E testnet integration scaffold (COW-1064)

[brunota20]

Summary

Wires the engine config, runbook, and report template for the
4-6 h E2E run on Sepolia with all 5 modules dispatched
simultaneously. This PR is scaffold-only — the run itself is
operator-driven (real Sepolia RPC keys, funded test EOA,
on-chain order submissions, 4-6 h wall clock) and cannot be
exercised from CI.

Stacks on #43 (COW-1073 multi-chain isolation). Linear: COW-1064.

What ships

File	What
engine.e2e.toml	Unified Sepolia config: 5 modules + [engine.metrics] enabled = true on 127.0.0.1:9100 + separate state_dir = ./data/e2e.
justfile	build-e2e (reuses build-m2 + build-m3) and run-e2e targets. JSON logs (no --pretty-logs) for jq mining.
docs/operations/e2e-testnet-runbook.md	Operator runbook: RPC selection, on-chain prep, boot expectations, the 3 on-chain triggers that satisfy the per-module terminal-markers acceptance, metrics capture, red flags, report-filing flow.
docs/operations/e2e-reports/e2e-report.template.md	Empty report skeleton with sections for metadata, chain coverage, submitted on-chain actions, per-module terminal markers, error-count deltas from /metrics, anomalies, acceptance checklist, sign-off (the gating decision for COW-1031).

What this PR does NOT do

The 4-6 h run itself is operator-driven and requires:

1. Real Sepolia RPC (Alchemy / Infura WS) — public node
   throttles eth_subscribe and eth_call under sustained
   load (≥ 4 calls/block from price-alert + balance-tracker ×2 +
   stop-loss + 1/registered-order from twap-monitor).
2. Funded test EOA + ComposableCoW Safe access on Sepolia —
   twap-monitor's only path to a submitted: marker.
3. EthFlow swap from that EOA on Sepolia —
   ethflow-watcher's only path to a submitted: marker.
4. setPreSignature + sell-token allowance from the stop-loss
   owner EOA — stop-loss's only path to a submitted:
   marker that is not a typed TransferSimulationFailed warn.
5. 4-6 h wall clock + metrics-start.txt / metrics-end.txt
   capture + the report's section 8 sign-off, which gates
   COW-1031 (7-day soak).

The runbook is explicit about each step; the report template's
acceptance checklist makes the closure conditions for COW-1064
unambiguous.

Smoke-validation done before commit

Booted engine.e2e.toml end-to-end against live Sepolia for
60+ s. Observed:

INFO supervisor ready modules=5 chains=1
INFO log subscription open module=twap-monitor chain_id=11155111
INFO block subscription open chain_id=11155111
INFO log subscription open module=ethflow-watcher chain_id=11155111
DEBUG dispatch ok module=twap-monitor    block_number=11088259 latency_ms=1
WARN  price-alert: TRIGGERED answer=168110190000 threshold=250000000000 (Below)
DEBUG dispatch ok module=price-alert     block_number=11088259 latency_ms=130
DEBUG dispatch ok module=balance-tracker block_number=11088259 latency_ms=271
WARN  stop-loss retry on next block (0): orderbook error
      (TransferSimulationFailed): sell token cannot be transferred
DEBUG dispatch ok module=stop-loss       block_number=11088259 latency_ms=1802

5/5 modules init successfully, both log subscriptions + the
block subscription open, dispatch loop ticks against real
Sepolia blocks, every block-subscribing module dispatches every
block, and the real RPC + Chainlink decode + cow-api submit
path is exercised inside seconds. Acceptance markers for
ethflow-watcher + the on-chain submitted: paths require the
operator's on-chain actions.

Workspace impact

• No production-code changes (pure ops scaffolding).
• cargo fmt --all --check clean.
• All 5 module artefacts build cleanly to the names
  engine.e2e.toml references (twap_monitor.wasm,
  ethflow_watcher.wasm, price_alert.wasm,
  balance_tracker.wasm, stop_loss.wasm).

AI assistance disclosure

Claude (Opus 4.7, 1M-context) drafted the runbook, report
template, and engine.e2e.toml from the existing M2 + M3
runbook shapes. I reviewed each file, ran the 60 s smoke boot
against live Sepolia (output above), and confirmed the
acceptance bar's "scaffold half" is complete. The "run half" I
will execute manually before pushing the COW-1064 report PR.

[linear-code]

COW-1064