Lean fixes async races and linter wording

ContextAsync.race no longer relies on sleep-based timing (ed0d50f)

The async race helper was reworked to resolve a promise from whichever task finishes first, then cancel the loser. This removes the old timing-sensitive behavior and should make ContextAsync-based code and tests far less flaky.

try? gets a test-only mode that skips built-in branches (6279ae9)

A new debug.tactic.try.onlyUserSuggestions option lets try? run only user-registered suggestion generators, bypassing expensive built-in branches like exact?, simp, and grind. That makes the resurrected cancellation tests faster and more stable, while also giving tests a cleaner way to exercise the suggestion pipeline.

unusedVariables now explains what to do instead (793cd14)

The linter message was rewritten from a terse "unused variable" warning to a more actionable explanation: the name is not explicitly referenced, and the binding can be removed or renamed to _ if used implicitly. This improves developer ergonomics and clarifies a common Lean warning.

Other misc changes

• Fixed toml_escape so empty internal leanc flags no longer emit moreLeancArgs = [""], avoiding a bogus empty argv entry in GCC builds. (bba3868)
• Updated tests for async cancellation, try?, and unused-variable diagnostics.