instrument: snap live params onto a fresh voice, roll a live drag back on capture loss, serialize the seqlock's two writers

src/core/instrument/engine/live_params.h

@@ -52,6 +52,16 @@ LiveValues foldLive(const PlayParams& params);
 // reader: after the retry budget it reports "nothing new" and the caller keeps its last good
 // snapshot rather than spinning on the audio thread.
 //
+// SINGLE-WRITER IS THE CALLER'S JOB and is load-bearing: two concurrent writers can leave the
+// generation EVEN mid-write (A stores gen+1, B reads odd and stores gen+2) while both copy the
+// block, and a reader then accepts a torn block as coherent. Every publisher must serialize.
+//
+// The plain (non-atomic) block copied across the fences is the standard pragmatic seqlock:
+// the fences give correct ordering, but the concurrent read of a non-atomic object is a data
+// race under the C++ object model, so TSan/UBSan will report it. That report is expected, not
+// a defect — there is no clean lock-free standard-C++ alternative that keeps the block a plain
+// value the audio thread can copy in one shot.
+//
 // The writer interface deliberately assumes NO particular thread beyond single-writer, so a
 // host's own parameter-change queue (delivered on the audio thread with sample offsets) can
 // drive it later without a redesign.
@@ -64,7 +74,10 @@ public:
         std::atomic_thread_fence(std::memory_order_release);
         values_ = values;
         std::atomic_thread_fence(std::memory_order_release);
-        seq_.store(gen + 2, std::memory_order_release);   // even: complete and coherent
+        // Skip 0 on wrap (~2^31 publishes): landing there would read as "never published" and
+        // stall every reader until the NEXT publish — a silent mode, unlike a loud one.
+        const std::uint32_t next = (gen + 2 == 0u) ? 2u : gen + 2;
+        seq_.store(next, std::memory_order_release);      // even: complete and coherent
     }
 
     // Copies the block into `out` and returns the generation actually observed, or 0 when