perf: add checkInterrupted to inferType cache miss path (#13258)

This PR adds a `Core.checkInterrupted` call in `checkInferTypeCache` on
cache miss, allowing cancellation to be detected during large type
inference traversals. Previously, `inferTypeImp` could run for >100ms
without any interruption check when processing large expressions (e.g.
BVDecide proof terms), making IDE cancellation unresponsive.

The check is only on cache miss (actual computation), so cache hits have
zero overhead. `checkInterrupted` is used instead of the heavier
`checkSystem` to minimize performance impact — local benchmarks show no
measurable regression on `tests/elab/5664.lean`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

src/Lean/Meta/InferType.lean

@@ -180,12 +180,14 @@ private def inferFVarType (fvarId : FVarId) : MetaM Expr := do
 
 @[inline] private def checkInferTypeCache (e : Expr) (inferType : MetaM Expr) : MetaM Expr := do
   if !(← read).cacheInferType || e.hasMVar then
+    Core.checkInterrupted
     inferType
   else
     let key ← mkExprConfigCacheKey e
     match (← get).cache.inferType.find? key with
     | some type => return type
     | none =>
+      Core.checkInterrupted
       let type ← inferType
       unless type.hasMVar do
         modifyInferTypeCache fun c => c.insert key type