feat: send EC shards in parallel per destination disk (#12050)

* feat: send EC shards in parallel per destination disk

One stream per destination disk. max_shard_transfers unset means no global cap, 0 sends shards sequentially, and N is a global cap that still keeps one stream per disk. The value is persisted through TaskPolicy. The maintenance form is a checkbox plus Max transfers because unset and 0 differ. The key is intentionally omitted from GetConfigSpec.

* ec shard distribution: treat negative max_shard_transfers as sequential

The config layer (normalizedMaxShardTransfers) clamps negative values to 0,
which selects sequential distribution. shardTransferCap diverged: a negative
cap that reached ShardDistributionOptions directly would have selected
uncapped parallel mode, the most aggressive setting. Align the options layer
with the same normalization so invalid input degrades to the conservative
mode.

* ec: serialize shard transfers to a server when a target leaves disk choice to the receiver

disk_id 0 is auto-placement on the receiving volume server, so an
auto-placed stream can land on a disk another queued stream is pinned
to. Grouping by (destNode, disk_id) alone let both run at once; collapse
all of that server's transfers under the destNode key instead.

* worker: clamp max_shard_transfers to the int32 wire range

ReadIntConfig returned the 0 fallback for values above MaxInt32, turning
a large positive cap into sequential mode. Read int64 and clamp into
range instead.

* worker: drop unused shard-transfer form helper

The maintenance form renders and collects max_shard_transfers in
plugin.templ JavaScript; the Go copy had no callers.

* worker: bound max_shard_transfers conversion under an in-range guard

---------

Co-authored-by: Chepurnov Simon <195125934+CHEPURNOV-S-S@users.noreply.github.com>
Co-authored-by: Chris Lu <chrislusf@users.noreply.github.com>

weed/command/scaffold/admin.toml

@@ -69,6 +69,9 @@
 # volume-slot deadband for shard placement: nodes whose free volume slots differ by at
 # most this much tie and fewer live shards wins; applied times data shards (0 = strict)
 # free_slots_deadband = 0
+# shard copy concurrency. Leave unset for one in-flight copy per destination disk.
+# 0 sends shards sequentially. A positive N is a global cap, still one per disk.
+# max_shard_transfers = 4
 # max retry attempts for a failed erasure coding job
 # retry_limit = 1
 # seconds to wait between retry attempts