feat(ecstore): add on-demand migration write-back pipeline (#7079)

* feat(ecstore): add on-demand migration pull queue and write-back pipeline

Background pull queue per bucket (bounded by pull_queue_capacity, concurrency via the state's pull slot), OdmWriteBack/PullSource traits, single-part and multipart write-back with a pumped body that enforces idle timeout, cancel and content length, retry policy for retryable source errors, inline commit helper, and stats accounting (rustfs/backlog#2153).

* feat(object): implement on-demand migration write-back over internal put

OdmWriteBack impl mapping source heads onto InternalPutContext (content-header allowlist, x-amz-meta copy, tags, dual-prefix odm-* provenance, ETag policy), injected into OnDemandMigrationSys at startup; removes the dead-code gates left by ODM-06a (rustfs/backlog#2153).

crates/ecstore/src/api/mod.rs

@@ -159,6 +159,11 @@ pub mod bucket {
             OnDemandMigrationConfig, OnDemandMigrationConfigError, PathStyle, PolicyConfig, Provider, RangeGetPolicy,
             SourceConfig, SourceCredentials, SourceErrorPolicy, SourceTimeout, TlsConfig, ValidationContext,
         };
+        pub use crate::bucket::on_demand_migration::{
+            EnqueueOutcome, LocalObject, MAX_MULTIPART_PARTS, OdmWriteBack, PULL_MAX_RETRIES, PULL_RETRY_BASE_DELAYS,
+            PullCompletion, PullQueue, PullReason, PullSource, SourceBody, WriteBackBody, WriteBackError, WriteBackOutcome,
+            WriteBackPart, WriteBackRequest, commit_inline, commit_inline_with,
+        };
         pub mod source_client {
             pub use crate::bucket::on_demand_migration::source_client::{
                 SourceClient, SourceClientSpec, SourceError, SourceGet, SourceHead, SourceObject, SourcePage, SourceProbe,