fix: add missing release() and adopt_lock_t to single-threaded unique_lock stub

[kjsdesigns]

This PR fixes runtime build issues when LEAN_MULTI_THREAD is not set.

Problem

When building with LEAN_MULTI_THREAD undefined (required for Emscripten/WASM targets), the stub unique_lock<T> in src/runtime/thread.h is missing two members that the real std::unique_lock provides:

1. release() — called by runtime code paths, causes a compile error when the stub is active
2. unique_lock(T const &, std::adopt_lock_t) — required by code that acquires a lock before constructing the unique_lock

The other stubs in this file (mutex, lock_guard, condition_variable) are complete; only unique_lock is missing API surface.

Fix

Add the two missing members to the single-threaded unique_lock stub:

unique_lock(T const &, std::adopt_lock_t) {}
T * release() { return nullptr; }

Both are no-ops, matching the semantics of a single-threaded environment. release() returns nullptr (no mutex to release). The adopt_lock_t constructor is a no-op (no lock to adopt).

Evidence

I've been using this fix in a production project (specify-lean) since v4.27.0 to build the Lean4 runtime to WASM via Emscripten. The fix has been stable across multiple Lean version bumps.

I posted about this on Zulip on 2026-03-21.

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 8b52f4e8f701a9b56a76e6e0e118ef587621c3e0 --onto fc0cf68539ad3b481a1802414c22c44506519c9d. You can force Mathlib CI using the force-mathlib-ci label. (2026-04-01 15:40:52)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 8b52f4e8f701a9b56a76e6e0e118ef587621c3e0 --onto cfa8c5a036d6990635c6ec50b02d0e806995cec3. You can force reference manual CI using the force-manual-ci label. (2026-04-01 15:40:54)