feat: support bundled Lean toolchain runtimes

[ejgallego]

This PR lets Lean release builders compile and install lean-beam for the exact toolchain being shipped, with a relocatable CLI, MCP server, daemon, and LSP plugin.

• Provide explicit build/install commands, compiler identity checks, installed provenance, and sessions bound to the Beam payload, while preserving installation from a checkout.
• Resolve CLI and MCP requests through the containing toolchain, honor declared selectors under inherited elan overrides, and reject foreign or incomplete runtimes without downloads or user setup changes.
• Keep native and checkout build outputs separate. Compile both OS helpers with the selected Lean compiler, native SDK headers, and toolchain-specific object traces.
• Exercise Linux x86_64/aarch64 and Windows through native jobs, and macOS through the combined platform job. Full macOS mode retains separate x86_64/aarch64 native jobs.
• Builds on Lean 4.33 support reduction (refactor: raise minimum Lean support to 4.33 #274), sync/readiness (fix: synchronize accepted Lean document state #268), MCP self-check lifecycle (fix: bound MCP self-check pipe I/O and shutdown #271), and configurable macOS CI (chore: consolidate macOS CI with configurable full coverage #275).