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/command/scaffold/admin.toml

@@ -64,6 +64,8 @@
 # preferred_tags = ["fast", "ssd"]
 # EC shard placement constraint, e.g. "020"; empty uses the master default replication
 # replica_placement = ""
+# fail planning when the placement constraints cannot be met instead of relaxing them
+# strict_placement = false
 # max retry attempts for a failed erasure coding job
 # retry_limit = 1
 # seconds to wait between retry attempts