Replace stale 'ethgild' submodule + remapping with 'rain.vats' canonical name

The repo still has \`lib/ethgild/\` (submodule tracking \`gildlab/ethgild\`, which redirects to \`rainlanguage/rain.vats\`) and \`ethgild/=lib/ethgild/src/\` in foundry.toml remappings. The legacy name shows up in every import statement across the codebase.

Migrate to the canonical name: deinit + re-add the submodule as \`lib/rain.vats\`, update the remapping to \`rain.vats/=lib/rain.vats/src/\`, and rewrite all \`ethgild/...\` imports.

Start with foundry.toml and the existing lib/ethgild submodule, then locate every ethgild/... import across the codebase. Re-add the dependency under lib/rain.vats, update the remapping to rain.vats/=lib/rain.vats/src/, rewrite all imports, and verify the repository still builds.

Written by the indexing model from the issue text.