feat: a cleared ring can resume its absolute count at a given origin

A derived ring that is emptied and refilled from its source's retained
window has to come back numbered as the source is. `clear()` restarts it
at 0, which leaves every absolute index -- `base()`, `produced()`,
`copy_from()` -- short by that origin for a whole window's worth of data.

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

include/plot/data/ring_buffer.hpp

@@ -127,6 +127,18 @@ class RingBuffer {
   /// pushing).
   void clear() noexcept { produced_.store(0, std::memory_order_relaxed); }
 
+  /// Consumer thread only: discards every element and resumes the absolute count
+  /// at `absolute`, so the next push() lands there (not safe while a producer is
+  /// pushing).
+  ///
+  /// This is for a *recompute*: a ring whose contents are derived from another is
+  /// emptied and refilled from a given absolute origin, and it must come back
+  /// numbered as its source is. clear() would restart it at 0 and leave every
+  /// absolute index -- base(), produced(), copy_from() -- off by that origin.
+  void clear_to(u64 absolute) noexcept {
+    produced_.store(absolute, std::memory_order_relaxed);
+  }
+
  private:
   u64 cap_;
   u64 mask_;