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/model.cpp

@@ -118,8 +118,11 @@ Partition plan_partition(const std::vector<double>& channel_dt, double buffer_by
   // Only now, with the ring as long as the budget can buy, decimate the shared clock
   // enough for the requested window to fit in it: deciding this earlier would over-
   // decimate by whatever factor the growth loop was about to add. A power of two keeps
-  // the ring capacities (themselves powers of two) in step and keeps `tick % period`
-  // cheap. The cap stops a nonsense request -- a window far past `--max-window` -- from
+  // the ring capacities (themselves powers of two) in step and lets the producer's group
+  // test be a mask (`Acquisition::acc_mask_`) rather than a modulo -- a modulo on a stride
+  // this function chose at runtime is a real 64-bit division, which the test *was* paying
+  // once per channel per wire sample.
+  // The cap stops a nonsense request -- a window far past `--max-window` -- from
   // spinning here; at 2^20 the stored rate is six decades below the wire rate and the
   // picture is averages of averages anyway.
   u64 m = 1;