build: delete the offline backtest harness

[mfw78]

What

Deletes crates/shepherd-backtest (the offline replay binary) and tools/backtest-collect (the 578-line collector and its 831K committed fixtures dump). Drops the crate from the workspace members and regenerates Cargo.lock; the lock delta is exactly the one removed package entry.

Drops rlib from ethflow-watcher's crate-type, which existed solely so the harness could drive strategy::on_chain_logs natively, leaving the wasm cdylib the engine actually loads. The cfg(not(target_arch = "wasm32")) gate in its lib.rs stays, because the crate is still compiled for the host as a workspace member; only the comment that justified it by naming the harness changed.

Clears the trailing references: the clap comment listing the harness as a consumer, the now-dead fixtures glob in .dockerignore, the Dockerfile and orderbook-mock comments, the nexum-sdk::address rationale, and the docs/00-overview.md crate tree.

Keeps the workspace hex dependency, which videre-test also uses, and keeps docs/operations/backtest-reports/backtest-7d-2026-06-22.md as the historical signed-off record.

Why

The watcher indexes from the contract's genesis at startup, so the live path already replays history. A frozen offline corpus re-asserting the same thing adds nothing, and no further backtest runs are planned.

The harness was also not earning its keep as code: it was in neither CI nor the justfile, had zero tests of its own, and produced exactly one report (window collected 2026-06-22) scoring 240/240 on a fixed corpus. It was nonetheless a workspace member that had to be repaired on every SDK rename, most recently in #470, where its imports still referenced the pre-rename nexum_sdk::status_body.

Two load-testnet-runbook.md bullets cited "the backtest" as covering appData and order-shape correctness. Rather than leave a dangling pointer, one now points at the genesis replay and the other states plainly that only a live-orderbook run exercises real 4xx variety.

Testing

Full local battery green: cargo fmt --all -- --check, cargo clippy --workspace --all-targets --all-features --locked -D warnings, all 18 guest wasms (17 modules plus the cow adapter), cargo nextest run --workspace --all-features --locked, cargo test --doc, RUSTDOCFLAGS=-D warnings cargo doc --workspace --no-deps --locked, scripts/check-venue-agnostic.sh, scripts/check-cow-orderbook-only.sh.

The wasm build is what confirms dropping rlib is safe: ethflow_watcher.wasm still builds and the count stays at 18.

AI Assistance

Implemented with Claude Code.