fix(ec): bring ec.encode worker and EC/volume helpers to parity with shell (#9599)

* refactor(volume): extract replica sync/select into shared volume_replica package

Move the volume replica reconciliation helpers (status, union builder,
SyncAndSelectBestReplica, ReadNeedleMeta) out of the shell into a new
weed/storage/volume_replica package so both the shell (ec.encode, volume.tier.move,
volume.check.disk) and the EC encode worker can reuse them. No behavior change.

* fix(ec): bring ec.encode worker to parity with the shell

- Sync replicas and encode the most-complete one (via the shared
  volume_replica.SyncAndSelectBestReplica) instead of a possibly-stale replica,
  marking all replicas readonly first. Prevents silent data loss when a stale
  replica is encoded and the originals deleted.
- Skip remote/tiered volumes in detection (shell ec.encode excludes them).
- Min-node safety gate: refuse to encode when cluster nodes < parity shards.
- Align default thresholds with the shell (fullness 0.95, quiet 1h).

* fix(vacuum): plugin path honors min_volume_age_seconds override

deriveVacuumConfig hard-coded MinVolumeAgeSeconds=0, dropping any configured
value. Read it from worker config (default 0, matching the shell/master vacuum
which has no age gate) so an explicit override is honored.

* address review feedback

- config.go: align GetConfigSpec schema defaults (quiet_for_seconds=3600,
  fullness_ratio=0.95) with the runtime defaults so UI/bootstrap flows match the
  shell (coderabbitai).
- ec_task.go: roll back readonly when markReplicasReadonly fails partway, so
  already-marked replicas don't stay readonly (coderabbitai).
- volume_replica: pass the caller's replica statuses into buildUnionReplica instead
  of re-fetching them, and skip the per-needle ReadNeedleMeta RPC when the source
  replica is read-only (gemini-code-assist).

* test(plugin_workers/ec): make fixtures eligible under the new defaults

The default EC encode thresholds were raised to match the shell (fullness 0.95,
quiet 1h), but the plugin-worker integration fixtures still used 90%-full /
10-minute-old volumes, so detection found no eligible volumes and the tests failed
in CI. Bump the eligible fixtures to 96% full and 2h old.

weed/worker/tasks/erasure_coding/detection_test.go

@@ -233,13 +233,13 @@ func buildStuckSourceTopology(t *testing.T, volumeID uint32, presentShardCount i
 // criteria (Age, FullnessRatio, Size), with `Age` derived from `LastModified`
 // so the two fields stay consistent for any reader.
 func buildStuckSourceMetrics(volumeID uint32, server string) []*types.VolumeHealthMetrics {
-	lastModified := time.Now().Add(-time.Hour)
+	lastModified := time.Now().Add(-2 * time.Hour)
 	return []*types.VolumeHealthMetrics{{
 		VolumeID:      volumeID,
 		Server:        server,
 		Size:          200 * 1024 * 1024,
 		Collection:    "",
-		FullnessRatio: 0.9,
+		FullnessRatio: 0.96,
 		LastModified:  lastModified,
 		Age:           time.Since(lastModified),
 	}}
@@ -482,9 +482,9 @@ func buildVolumeMetricsForIDs(count int) []*types.VolumeHealthMetrics {
 			Server:        "10.0.0.1:8080",
 			Size:          200 * 1024 * 1024,
 			Collection:    "",
-			FullnessRatio: 0.9,
-			LastModified:  now.Add(-time.Hour),
-			Age:           10 * time.Minute,
+			FullnessRatio: 0.96,
+			LastModified:  now.Add(-2 * time.Hour),
+			Age:           2 * time.Hour,
 		})
 	}
 	return metrics