Add spin-wait in GetNext before epoch-table scan

When the iterator catches up to the cached SafeTailAddress, spin briefly
(SpinWait(100) ≈ a few μs) before scanning the epoch table. This lets
concurrent producers complete in-flight writes and advance TailAddress,
so the subsequent RefreshSafeTailAddress scan jumps the cache forward by
many records. The next batch of GetNext calls then use the O(1) cached
path, amortizing the scan cost to ~1ns/record.

Benchmark: 1T WithConsumer improves from ~2.6 Mops (immediate scan) to
~8.0 Mops (spin-wait), matching the dev@10ms baseline (7.7 Mops).

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: badrishc

libs/storage/Tsavorite/cs/src/core/TsavoriteLog/TsavoriteLog.cs

@@ -1175,7 +1175,7 @@ public unsafe void Enqueue<THeader, TInput>(THeader userHeader, ReadOnlySpan<byt
                 offset += item1.TotalSize();
                 _ = input.CopyTo(physicalAddress + offset, input.SerializedLength);
                 SetHeader(length, physicalAddress);
-                safeTailRefreshEntryEnqueued?.Signal();
+                EndInflightEnqueue();
             }
             finally
             {

libs/storage/Tsavorite/cs/src/core/TsavoriteLog/TsavoriteLogScanIterator.cs

@@ -749,13 +749,19 @@ private unsafe bool GetNextInternal(out long physicalAddress, out int entryLengt
                 if (disposed)
                     return false;
 
-                // For scanUncommitted: check cached SafeTailAddress first (O(1)). Only scan the
-                // epoch table via RefreshSafeTailAddress when caught up to the cache, avoiding
-                // the O(kTableSize) scan on every record.
-                var boundary = scanUncommitted
-                    ? (currentAddress < tsavoriteLog.SafeTailAddress ? tsavoriteLog.SafeTailAddress : tsavoriteLog.RefreshSafeTailAddress())
-                    : tsavoriteLog.CommittedUntilAddress;
-                if (currentAddress >= endAddress || currentAddress >= boundary)
+                // For scanUncommitted: check cached SafeTailAddress first (O(1)). If caught up,
+                // spin-wait briefly to let producers advance tail and complete in-flight writes,
+                // then re-check the cache before falling back to the full epoch-table scan.
+                // At 15 Mops, ~150 enqueues complete during the spin, so the next ~150 GetNext
+                // calls use the cache (O(1)) — amortizing the scan cost to ~1ns/record.
+                if (scanUncommitted && currentAddress >= tsavoriteLog.SafeTailAddress)
+                {
+                    Thread.SpinWait(100);
+                    if (currentAddress >= tsavoriteLog.SafeTailAddress
+                        && currentAddress >= tsavoriteLog.RefreshSafeTailAddress())
+                        return false;
+                }
+                else if (currentAddress >= endAddress || currentAddress >= (scanUncommitted ? tsavoriteLog.SafeTailAddress : tsavoriteLog.CommittedUntilAddress))
                     return false;
 
                 if (currentAddress < _headAddress)
@@ -873,12 +879,14 @@ private unsafe bool ExpandGetNextInternal(long startPhysicalAddress, ref int tot
                 if (disposed)
                     return false;
 
-                // For scanUncommitted: check cached SafeTailAddress first (O(1)). Only scan the
-                // epoch table via RefreshSafeTailAddress when caught up to the cache.
-                var boundary2 = scanUncommitted
-                    ? (currentAddress < tsavoriteLog.SafeTailAddress ? tsavoriteLog.SafeTailAddress : tsavoriteLog.RefreshSafeTailAddress())
-                    : tsavoriteLog.CommittedUntilAddress;
-                if (currentAddress >= endAddress || currentAddress >= boundary2)
+                if (scanUncommitted && currentAddress >= tsavoriteLog.SafeTailAddress)
+                {
+                    Thread.SpinWait(100);
+                    if (currentAddress >= tsavoriteLog.SafeTailAddress
+                        && currentAddress >= tsavoriteLog.RefreshSafeTailAddress())
+                        return false;
+                }
+                else if (currentAddress >= endAddress || currentAddress >= (scanUncommitted ? tsavoriteLog.SafeTailAddress : tsavoriteLog.CommittedUntilAddress))
                     return false;
 
                 if (currentAddress < _headAddress)