[tooling] anvil_fork.sh always forks at 'latest' — demo non-reproducible, 100-block soak has no baseline

[obchain]

Refs #52

File: scripts/anvil_fork.sh

Problem

Script forks at 'latest'. Every run is against a different block. The PR defers the 100-block soak test — but without block pinning, the soak has no reproducible baseline. Fork-test suite in PR #53 will also struggle without pinned scenarios.

Fix

Accept optional --fork-block-number flag:

FORK_BLOCK="${FORK_BLOCK:-latest}"
anvil --fork-url "$RPC" --fork-block-number "$FORK_BLOCK" --chain-id 56 --block-time 3

Document recommended pin blocks in README (one for Grafana demo, one for each fork-test scenario).

[obchain]

Fixed on feat/24-anvil-fork (ffae806): DEFAULT_FORK_BLOCK=94_000_000, FORK_BLOCK=latest available as opt-in.