ec: recover EC shards whose .ecx index lives only on a peer server (#10108)

* ec: recover EC shards whose .ecx index lives only on a peer server

A volume server that boots with EC shard files on disk but no .ecx index
on any local disk cannot mount the shards, so the master never learns
about them. ec.rebuild works off master-registered shards, so it sees the
volume as short and gives up even though the shard data is intact.

Add an operator-triggered recovery: VolumeEcShardsMount gains a
recover_missing_index flag that makes the volume server fetch the missing
.ecx (plus .ecj/.vif) from a peer holding it and mount the on-disk shards.
ec.rebuild runs this across the cluster before planning, so orphaned
shards register and the rebuild sees the true shard set.

.ecx is an immutable encode-time index, identical on every holder. .ecj
is a per-holder deletion journal that differs across holders, so the
recovered node adopts the source peer's deletion view, like a balanced or
rebuilt shard does.

* ec: mirror missing-index recovery into the Rust volume server

Port the #10104 recovery to seaweed-volume so the Rust volume server
self-heals the same layout: EC shards on disk with the .ecx index only on
a peer. Adds collect_ec_volumes_missing_index / mount_recovered_ec_shards
to the store, recover_missing_ec_indexes (master LookupEcVolume + peer
CopyFile fetch + mount) to the server, and the recover_missing_index flag
on VolumeEcShardsMount.

.ecx is the immutable encode-time index, identical on every holder. .ecj
is a per-holder deletion journal, so the recovered node adopts the source
peer's deletion view, matching the Go path.

weed/pb/volume_server.proto

@@ -464,6 +464,7 @@ message VolumeEcShardsMountRequest {
     string collection = 2;
     repeated uint32 shard_ids = 3;
     string source_disk_type = 4; // disk type of the source volume, applied to the in-memory EC volume so heartbeats report under it (#9423)
+    bool recover_missing_index = 5; // first fetch a missing .ecx index from a peer so on-disk shards without a local index become mountable (#10104)
 }
 message VolumeEcShardsMountResponse {
 }