v0.2.3

lean-rs-abi is purely static; toolchain probing moves to the crates that link or discover Lean

lean-rs-abi no longer has a build script. Previously its build.rs probed an installed Lean toolchain (hashing
lean.h, reading the version) and panicked when none was found, so a crate documented as "link-free metadata" could not
be built by any pure-Rust consumer without a Lean toolchain on PATH (or DOCS_RS=1) — which kept downstream Rust-only
CI red. The crate is now purely static: the SUPPORTED_TOOLCHAINS window, REQUIRED_SYMBOLS, and the lean.h layout
literals, with no probe.

The live toolchain identity it used to bake (LEAN_VERSION, LEAN_RESOLVED_VERSION, LEAN_HEADER_PATH,
LEAN_HEADER_DIGEST) moves to the crates that legitimately have a toolchain at build time:

• lean-rs-sys (which links libleanshared) now bakes these from its own build script and exposes them at the same
  paths (lean_rs_sys::LEAN_VERSION, lean_rs_sys::consts::*).
• lean-toolchain (the discovery crate) now bakes them in its own build.rs and re-exports them at the same paths
  (lean_toolchain::LEAN_VERSION, …). Its probe degrades to the latest supported window entry when no toolchain is
  present instead of failing, so link-free downstream crates (and docs.rs) build with no Lean installed. It still
  fails the build when an installed toolchain's lean.h is outside the supported window.

lean_rs_abi::LEAN_VERSION, LEAN_RESOLVED_VERSION, LEAN_HEADER_PATH, and LEAN_HEADER_DIGEST are removed; consume
them from lean-rs-sys or lean-toolchain instead. No other public API or layout change.