volume: EC decode onto a clean peer via staged-new-volume adopt (Go+Rust) (#10463)

Decoding EC shards back to a normal volume in place reconstructs <vid>.dat
in the shards' own directory, so the vid is momentarily registered as both
an EC and a normal volume in one location — the load/scan path then sees it
as both, risking mount ambiguity and needle loss. VolumeEcShardsToVolume
still supports that in-place path; this adds the primitives to decode onto
a *clean* peer instead:

  - ReceiveFile gains a staged-new-volume mode: when the volume does not
    exist here and ReceiveFileInfo.disk_type is set, pick a free-slot disk
    of that medium and write <base><ext>.copying (not a valid volume name,
    so the scanner never half-loads a partial push).
  - VolumeEcShardsToVolume gains from_staged: adopt the pushed .dat/.idx/
    .vif — rename .copying into place under a .note in-progress marker,
    then mount — so <vid> lands on the peer only as a normal volume.

The caller decodes the shards off-box and streams the finished volume to a
peer holding no shard of the vid on the target medium. Go and Rust volume
servers get identical handlers. Proto: ReceiveFileInfo.disk_type (12; 8-11
reserved for versioned-EC), VolumeEcShardsToVolumeRequest.from_staged (3) +
disk_type (4).

Claude-Session: https://claude.ai/code/session_01Ks16jnt4S7gdDk8cheQ3xu

weed/pb/volume_server.proto

@@ -351,6 +351,13 @@ message ReceiveFileInfo {
     uint32 shard_id = 5;
     uint64 file_size = 6;
     uint32 disk_id = 7;  // EC shard disk; 0 = auto-select (see VolumeEcShardsCopyRequest.disk_id)
+
+    // Field numbers 8-11 are reserved for versioned-EC; disk_type stays at 12.
+    // Staged-new-volume mode (EC decode onto a clean peer): set on a non-EC push
+    // whose volume does not yet exist on this server. The server picks a disk
+    // location of this medium with a free slot and writes <base><ext>.copying,
+    // finalized by VolumeEcShardsToVolume(from_staged).
+    string disk_type = 12;
 }
 
 message ReceiveFileResponse {
@@ -512,6 +519,13 @@ message VolumeEcBlobDeleteResponse {
 message VolumeEcShardsToVolumeRequest {
     uint32 volume_id = 1;
     string collection = 2;
+    // Staged mode: the caller already decoded the EC shards off-box and pushed
+    // .dat/.idx/.vif as <base><ext>.copying to this server (ReceiveFile
+    // staged-new-volume mode). Adopt them as a normal volume instead of decoding
+    // local EC shards in place, so <vid> is never registered as both EC and
+    // normal on one disk.
+    bool from_staged = 3;
+    string disk_type = 4; // target medium's disk location for the normal volume (staged mode)
 }
 message VolumeEcShardsToVolumeResponse {
 }