measurement: adapt the echo probe rate to loss and RTT, and weight samples by the time they stand for

udp_echo and tcp_echo run at the lowest rate that still proves the loss bound and keeps the RTT p99 confidence interval within tolerance, go to the maximum when the line loses packets or its RTT rises, and back off exponentially when it is unreachable. Every echo row carries a weight (the time it stands for, in grid intervals) and the analysis ops accumulate loss and RTT by it, so p50/p95/p99 and loss stay time-averaged when the rate changes. State changes are logged for audit.

backend/shared/tests/MoShiro.Protocol.Tests/RawChunkCodecTests.cs

@@ -92,6 +92,7 @@ public sealed class RawChunkCodecTests
     public void GoldenBody_LayoutIsStable()
     {
         // 布局变化（列编码、头部字段、字节序）会让这个测试失败：必须同时升级 container_version 并更新黄金字节。
+        // 只是回显流多了一列权重 w（块头自描述新增列，不用升级 container_version），黄金字节里多出六个 01。
         RawChunk chunk = SampleChunk();
         chunk.Header.Clock = new ChunkClock { ListenerMonoUs = 1000, UtcMs = 1_700_000_000_000, DialerOffsetUs = 500 };
         byte[] body = RawChunkCodec.EncodeBody(chunk);
@@ -104,7 +105,7 @@ public sealed class RawChunkCodecTests
         Assert.Equal(GoldenColumns, columns);
     }
 
-    private const string GoldenColumns = "00020202020240404040404080fbf3b407c09a0cc09a0cc09a0cc09a0cc09a0c3bc0e0d8b907889c0c90b818889c0c889c0c3ba4e1d8b907889c0c90b818889c0c889c0c2bb4fcf8b407889c0c90b81890b818";
+    private const string GoldenColumns = "00020202020240404040404001010101010180fbf3b407c09a0cc09a0cc09a0cc09a0cc09a0c3bc0e0d8b907889c0c90b818889c0c889c0c3ba4e1d8b907889c0c90b818889c0c889c0c2bb4fcf8b407889c0c90b81890b818";
 
     [Fact]
     public void Decode_RejectsBadMagicVersionCrcAndTruncation()