feat: a late packet backfills the hole it left, values only

src/channel.hpp

@@ -49,6 +49,17 @@ class ChannelBuffer final : public plot::data::Series {
   /// Consumer thread: forgets the published window (after a rebind or a reset).
   void invalidate() noexcept;
 
+  /// Producer thread, under `ScopeModel::mutex()`: records that absolute sample `absolute`
+  /// has been backfilled, for `acquire()` to fold into the pyramid.
+  ///
+  /// The pyramid is mutated *only* on the consumer thread, because a bucket is a 32-byte
+  /// non-atomic aggregate the renderer reads unlocked at paint time: amending it from the
+  /// producer would let a reader see a half-updated bucket whose `sum` did not match its
+  /// `count`, which is a wrong mean out of a structure whose readers trust `count == 0` to
+  /// mean absent. The value ring is amended by the producer, which owns its writes; the
+  /// pyramid waits for here.
+  void queue_amend(u64 absolute, f32 v) { pending_amends_.push_back({absolute, v}); }
+
   void set_nominal_dt(f64 dt) noexcept { dt_ = dt; }
   /// Re-bases the x axis: `x0()`/`x_at()` become `epoch + timestamp`. Used to make
   /// the axis read seconds from the trigger event instead of from the stream.
@@ -100,6 +111,13 @@ class ChannelBuffer final : public plot::data::Series {
   /// Timestamp of channel sample `absolute_index`, in seconds since `epoch_`.
   f64 time_of(u64 absolute_index) const noexcept;
 
+  struct Amend {
+    u64 absolute;
+    f32 value;
+  };
+  /// Backfills the producer has written into the value ring and the pyramid has not seen yet.
+  std::vector<Amend> pending_amends_;
+
   std::shared_ptr<const plot::data::RingBuffer<f32>> values_;
   std::shared_ptr<const plot::data::RingBuffer<f64>> times_;
   u64 stride_ = 1;