[david415]

Moves the Lean client to Lean v4.35.0, the latest stable release.

• lean-toolchain v4.35.0; the lakefile's version 0.2.0 (it was 0.1.0, while this repository's Lean tags reach v0.0.16: which scheme to tag with is still to settle).
• Pins: cborg v0.2.0; cryptlib at Lean 4.35.0; version 0.2.0 cryptlib#53 and lean_unix_socket at Lean 4.35.0 lean_unix_socket#4, both on 4.35.0, to be repinned at their merges.
• cryptlib renamed its lowercase modules when it was prepared for Reservoir: Cryptlib.Util.newhex is Cryptlib.Util.HexBytes and Cryptlib.Protocols.Sphinx.geometry_cbor is …GeometryCbor; the imports follow.

Draft until cryptlib#53 and lean_unix_socket#4 merge; lean_unix_socket#4 waits for katzenpost-lean-sys 0.1.0 on crates.io.

Locally (lean_unix_socket's crate against katzenpost-lean-sys by a path patch; Mathlib from its cache): make test-lean passes, with all 24 requests decoding as the daemon's types; thinclient-integration builds.

[gabrix73]

Reviewed at 985dc6a. The Lean toolchain update, dependency pins, and import changes look good; make test-lean passes in CI. Please align the cryptlib and lean_unix_socket pins with their merged dependency PRs (#53 and #4) before merging this PR.