ec.encode: require shard placement to meet -shardReplicaPlacement (#11947)

* ec: measure shard placement against replica-placement caps

ecShardPlacementViolations reports where a volume's newest-generation
shard placement exceeds the per-node and per-rack caps a
-shardReplicaPlacement requests, using the same digit mapping the
balance planner enforces (SameRackCount = shards/node, DiffRackCount =
shards/rack, dc-qualified rack keys).

* ec.encode: -strictPlacement keeps originals when placement breaches caps

-shardReplicaPlacement tells the balancer the requested per-node and
per-rack shard limits, but nothing verifies the outcome. With
-strictPlacement the pre-delete shard verification also measures the
final placement against those caps: a volume still over the node or
rack limit aborts before its source is deleted, so the encode only
completes when the requested failure-domain spread is actually met.

* ec.encode: cover the strict-placement delete gate with a test

The gate in verifyEcShardsBeforeDelete now runs against a stubbed
topology: a compliant spread passes while an over-cap placement is
refused before source deletion. The retry knobs become package vars so
the test does not wait out the heartbeat-propagation backoff.

* ec.encode: enforce -shardReplicaPlacement without a flag

Drop the -strictPlacement option: the requested per-node and per-rack
caps are always requirements at the pre-delete verification, so the
encode only completes when the placement actually honors them.

weed/shell/command_ec_encode.go

@@ -54,7 +54,8 @@ func (c *commandEcEncode) Help() string {
 	per node (the data-center digit is not used for EC). For example
 	-shardReplicaPlacement=021 requests at most 1 shard per node and 2 per rack.
 	Only when the final placement meets these limits does losing one rack cost
-	at most 2 shards; the command does not guarantee that the limits are met.
+	at most 2 shards, so the encode fails (and keeps the original volumes)
+	when the cluster cannot honor them.
 
 	The -collection parameter is a comma-separated list of collection names, with
 	"*" and "?" wildcards, and regex patterns: