ec: strict_placement option so encode only runs while guarantees hold (#11656)

* ec: strict_placement option so encode only runs while guarantees hold

Shard placement during encode was best-effort (PlaceDurabilityFirst):
when the cluster could not satisfy the per-disk caps, anti-affinity,
replica-placement or per-rack caps, the constraints were relaxed and
the volume was encoded anyway, weaker than configured. A
strict_placement option on the erasure coding task switches planning
to PlaceStrict so the volume's planning fails instead, and the encode
is retried when capacity allows the guarantee.

Also documents the resilience rule in ec.encode help: a volume
survives losing any nodes or racks holding at most parity-shards
shards between them, and how -shardReplicaPlacement's rack and node
digits bound that loss.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

* ec: expose strict_placement through the plugin form and persisted task policy

The admin UI, the admin.toml maintenance mapping, and the TaskPolicy
serialization all dropped the new flag; add the bool field to
ErasureCodingTaskConfig, the worker config form, and both conversion
directions.

* shell: describe shardReplicaPlacement as requested limits, not guarantees

ec.encode places shards best-effort, so the configured rack/node caps only
bound shard loss when the final placement actually satisfies them.

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Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

---------

Co-authored-by: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>

weed/shell/command_ec_encode.go

@@ -47,6 +47,15 @@ func (c *commandEcEncode) Help() string {
 	If you only have less than 4 volume servers, with erasure coding, at least you can afford to
 	have 4 corrupted shard files.
 
+	The guarantee follows from where shards land: a volume survives the loss of any
+	nodes (or racks) that hold at most parityShards (4) shards between them. Spread
+	is best-effort; -shardReplicaPlacement requests limits: its rack digit
+	sets the requested shards per rack, and its node digit the requested shards
+	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.
+
 	The -collection parameter is a comma-separated list of collection names, with
 	"*" and "?" wildcards, and regex patterns:
 	  - One collection: ec.encode -collection="mybucket"