feat: add checkSystem calls to bv_decide for cancellation responsiveness (#13284)

This PR adds `checkSystem` calls to the `bv_decide` tactic's main loops
to improve IDE cancellation responsiveness. Previously, `bv_decide` had
zero `checkSystem` calls in its entire codebase, meaning long-running
invocations could not be cancelled promptly.

Insertion points:
- `fixpointPipeline` (normalization fixpoint loop)
- `ReifiedBVExpr.of.go` (recursive BitVec expression reification)
- `ReifiedBVLogical.of.go` (recursive boolean expression reification)
- `reflectBV` (hypothesis loop)

These cover the MetaM-based phases of `bv_decide`. The pure computation
phases (bitblasting via `IO.lazyPure`, AIG→CNF conversion) remain
without checks and would require restructuring to address.

Gap measurements (at 6.0 Ginstr/s, using
`LEAN_CHECK_SYSTEM_INTERVAL_INSN` from #13212):
- `bv_decide_large_aig.lean`: before ~3.3s max gap (19,899M insn),
after: pure bitblast/AIG gaps remain but reification and normalization
phases are now interruptible
- Other bv_decide bench files: normalization and reification loops are
now responsive to cancellation

Note: the largest remaining gaps in bv_decide benchmarks are in the
kernel type checker (proof term checking, up to 10.5s on
`bv_decide_mod.lean`) and in pure bitblasting — both out of scope for
this PR.

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

src/Lean/Elab/Tactic/BVDecide/Frontend/BVDecide.lean

@@ -357,6 +357,7 @@ def reflectBV (g : MVarId) : M ReflectionResult := g.withContext do
   let mut sats := #[]
   let mut unusedHypotheses := {}
   for hyp in hyps do
+    checkSystem "bv_decide"
     if let (some reflected, lemmas) ← (SatAtBVLogical.of (mkFVar hyp)).run then
       sats := (sats ++ lemmas).push reflected
     else

src/Lean/Elab/Tactic/BVDecide/Frontend/BVDecide/Reify.lean

@@ -33,6 +33,7 @@ where
   Reify `x`, returns `none` if the reification procedure failed.
   -/
   go (origExpr : Expr) : LemmaM (Option ReifiedBVExpr) := do
+    checkSystem "bv_decide"
     match_expr origExpr with
     | BitVec.ofNat _ _ => goBvLit origExpr
     | HAnd.hAnd _ _ _ _ lhsExpr rhsExpr =>
@@ -340,6 +341,7 @@ where
   Reify `t`, returns `none` if the reification procedure failed.
   -/
   go (origExpr : Expr) : LemmaM (Option ReifiedBVLogical) := do
+    checkSystem "bv_decide"
     match_expr origExpr with
     | Bool.true => ReifiedBVLogical.mkBoolConst true
     | Bool.false => ReifiedBVLogical.mkBoolConst false

src/Lean/Elab/Tactic/BVDecide/Frontend/Normalize/Basic.lean

@@ -159,6 +159,7 @@ Repeatedly run a list of `Pass` until they either close the goal or an iteration
 the goal anymore.
 -/
 partial def fixpointPipeline (passes : List Pass) (goal : MVarId) : PreProcessM (Option MVarId) := do
+  checkSystem "bv_decide"
   let mut newGoal := goal
   for pass in passes do
     if let some nextGoal ← pass.run newGoal then