fix: align Beam runtimes with Elan toolchain selection

[ejgallego]

This PR follows the effective Elan workspace selection and uses its concrete toolchain and fingerprinted compiler installation for the Beam runtime, preventing plugin mismatches under overrides and moving channels.

• Honor environment and directory overrides through the Elan 4.0 JSON interface; resolve channels once before admission, using the installed fallback only after a live lookup failure.
• Reject moving selectors as custom runtime identities and report the original selector alongside the concrete toolchain in doctor.
• Reuse the compiler installation already fingerprinted for the bundle instead of selecting an executable again.
• Check the minimum Elan interface in CI and reject unavailable interfaces before writing install state.
• Keep installer and defensive validation clones independent of their source checkout Git metadata.