perf: the producer's group test is a mask, not a 64-bit division

`plan_partition()` only ever chooses powers of two for `time_stride`, and the
comment there already claimed the test was cheap -- but the stride is a runtime
value, so `tick % period` compiled to a real division that the hot loop paid
once per channel per wire sample. The per-channel masks are built where the
partition is already reacted to, and the shared clock's group end is computed
once per sample instead of three times.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/acquisition.hpp

@@ -180,6 +180,15 @@ class Acquisition {
   /// once the slot's samples have all been pushed. One entry per channel, refilled every slot.
   std::vector<f32> math_sample_;
   std::vector<std::uint64_t> value_acc_n_;
+  /// Per channel, `time_stride * stride[i] - 1`: the group boundary as a **mask**, so the
+  /// innermost loop tests `(tick & mask) == mask` instead of `tick % period == period - 1`.
+  /// The divisor is a runtime value, so that modulo compiled to a real 64-bit `div` once per
+  /// channel per *wire* sample -- the filter reads every sample by design, so it was the
+  /// producer's largest avoidable cost. Both factors are powers of two (`plan_partition()`
+  /// doubles `time_stride` from 1, and every `stride[i]` is 1), which is what makes the mask
+  /// equivalent; it is sized and filled with the accumulators below, so a re-plan cannot
+  /// leave it addressing the previous partition's groups.
+  std::vector<std::uint64_t> acc_mask_;
   double time_acc_ = 0.0;
   double abs_time_acc_ = 0.0;
   std::uint64_t time_acc_n_ = 0;