[tooling] Port 8545 hardcoded in script and fork.toml — parallel runs, CI matrix impossible

[obchain]

Refs #52

Files: scripts/anvil_fork.sh, config/fork.toml

Problem

anvil default port 8545 is hardcoded on both sides. Multiple concurrent forks (e.g. two test suites in CI, or a dev running fork while CI runs fork tests) collide. No way to parameterise.

Fix

Accept PORT env var:

PORT="${ANVIL_PORT:-8545}"
anvil --port "$PORT" ...

config/fork.toml uses ${ANVIL_PORT:-8545} substitution. README documents override mechanism.

[obchain]

Fixed on feat/24-anvil-fork (fe60699): config/fork.toml now reads ${CHARON_ANVIL_PORT:-8545} on both ws_url and http_url. substitute_env_vars in crates/charon-core/src/config.rs grew POSIX-style :-default expansion with unit coverage for set, unset-with-default, unset-without-default (still errors), empty-with-default, overridden, and unterminated cases. anvil_fork.sh reads the same CHARON_ANVIL_PORT (commit 4438673), with FORK_PORT kept as a legacy alias.