Name the collision: two messages, one name

The pin cited etcdpb.CollectionShardInfo without saying it is a different
message from this one. They share a name -- one in the collection meta with
a single truncate tick, introduced by the truncate API and on master today;
this one new here with four fields -- so a reader takes the sentence as
self-reference and the pin as arbitrary.

It is not: the collection meta swaps that message for this one at the same
field number, and records written before the swap must still decode. Said
outright now, collision included.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Signed-off-by: Cai Zhang <cai.zhang@zilliz.com>

Author: xiaocai2333

go-api/schemapb/schema.pb.go

@@ -591,13 +591,14 @@ enum ShardState {
 // and that belongs to the split task, which ends; the shards do not.
 message CollectionShardInfo {
   // The time tick this shard's WAL is truncated up to, 0 if never. Unrelated to
-  // routing, and older than it: the truncate API already kept a per-shard
-  // record next to the vchannel list, as etcdpb.CollectionShardInfo with this
-  // one field. This message replaces that one, adding the state and predicate a
-  // split needs beside the truncate tick that was already there.
+  // routing, and older than it.
   //
-  // Which is why the number must stay 1 -- it decodes the bytes that record
-  // already persisted.
+  // The truncate API already kept a per-shard record beside the vchannel list,
+  // in a SEPARATE message that happens to share this one's name --
+  // etcdpb.CollectionShardInfo, holding this field and nothing else. The
+  // collection meta swaps that message for this one at the same field number,
+  // so records written before the swap must still decode: hence field 1 here
+  // is that same truncate tick, and cannot be renumbered.
   uint64 last_truncate_time_tick = 1;
   ShardState state = 2;
   // The shard's own vchannel, so a consumer can key by it rather than by