perf(qwp): fuse per-frame CRC into one native call in MmapSegment.tryAppend

The append hot path computed the frame CRC-32C with two Crc32c.update
JNI calls -- one over the u32 payloadLen field, one over the payload --
even though the layout [u32 crc][u32 payloadLen][payload] places the
payload immediately after payloadLen (at lenAddr+4). The two CRC'd
regions are physically contiguous, so a single update over 4 + payloadLen
bytes yields the byte-identical CRC while crossing the JNI boundary once
per frame instead of twice.

This is the same form the recovery scanner already recomputes
(reader.crc32c(pos + 4, 4 + payloadLen) in scanForRecovery), which
validates every on-disk frame against the producer-written CRC, so the
fused value is guaranteed identical and the round-trip is unchanged.

No behavior change, no change to hashed or written bytes; saves one
native-call dispatch per non-empty frame on the producer's zero-alloc
hot path. Verified by MmapSegmentTest and MmapSegmentRecoveryFaultTest.

Author: bluestreak01

core/src/main/java/io/questdb/client/cutlass/qwp/client/sf/cursor/MmapSegment.java

@@ -554,16 +554,17 @@ public long tryAppend(long payloadAddr, int payloadLen) {
             return -1L;
         }
         // CRC32C over the (payloadLen, payload) pair. Recovery scans validate
-        // each frame by recomputing this CRC over the on-disk bytes.
+        // each frame by recomputing this CRC over the on-disk bytes. The
+        // payloadLen field (4 bytes at lenAddr) and the payload (at lenAddr+4)
+        // are physically contiguous, so a single CRC pass covers both -- one
+        // native call per frame, byte-identical to the chained form the
+        // recovery scanner recomputes (see scanForRecovery).
         long lenAddr = mmapAddress + offset + 4;
         Unsafe.getUnsafe().putInt(lenAddr, payloadLen);
         if (payloadLen > 0) {
             Unsafe.getUnsafe().copyMemory(payloadAddr, mmapAddress + offset + FRAME_HEADER_SIZE, payloadLen);
         }
-        int crc = Crc32c.update(Crc32c.INIT, lenAddr, 4);
-        if (payloadLen > 0) {
-            crc = Crc32c.update(crc, mmapAddress + offset + FRAME_HEADER_SIZE, payloadLen);
-        }
+        int crc = Crc32c.update(Crc32c.INIT, lenAddr, 4L + payloadLen);
         Unsafe.getUnsafe().putInt(mmapAddress + offset, crc);
         appendCursor = offset + total;
         // Plain read + write of the volatile field. `frameCount++` would