* feat: Free Volume Deadband (M) for EC Place nodes and racks
|ΔfreeSlots| ≤ M is a tie; remainder is liveShards. M=0 stays strict
freeSlots. M is free volume slots (× dataShards). pickNodeInRackEligible
and pickRackEligible share the rule. Worker plugin config
free_slots_deadband, default 0. Not on the write path.
* fix: anchor EC freeSlots deadband on class maxFree
Pairwise |Δfree|≤M vs the current winner is not transitive: 120→110→100
at M=10 slots picked 100. After evenness, rank by maxFree, keep
|free-maxFree|≤M, then min liveShards. Shared by node and rack. M=0 is
still only maxFree then live.
* worker: add free_slots_deadband to ErasureCodingTaskConfig
* erasure_coding: persist free_slots_deadband through TaskPolicy
ApplyMaintenanceConfigFromToml accepted the key but the admin save path
round-trips config through ToTaskPolicy, so the value was dropped on
reload; serialize it like the other EC settings.
* erasure_coding: bound free_slots_deadband at its declared maximum
The TOML loader and the plugin worker config only clamped negatives, so
a value above the spec's 100000 could reach the int32 TaskPolicy field
and wrap; cap at 100000 on both entry paths and declare MaxValue on the
plugin field.
* erasure_coding: document free_slots_deadband in scaffold admin.toml
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Co-authored-by: Chepurnov Simon <195125934+CHEPURNOV-S-S@users.noreply.github.com>
Co-authored-by: user.email <chrislusf@users.noreply.github.com>
