refactor(ecstore): move object_lock WORM evaluation onto storage-level types (#6666)

The object_lock module evaluated WORM state through s3s wire DTOs (ObjectLockRetention, ObjectLockLegalHold, DefaultRetention, Date) and s3s header constants, keeping the storage engine coupled to the serving protocol (rustfs/backlog#1842, ARCHITECTURE.md invariant 4). This PR gives the module its own storage-level vocabulary and pushes the DTO conversions to the boundaries that already speak s3s.

New crates/ecstore/src/bucket/object_lock/types.rs defines RetentionMode, LegalHoldStatus, ObjectRetention, ObjectLegalHold, and DefaultRetention with no s3s dependency. objectlock.rs parses persisted metadata into these types using the rustfs-utils lowercase header constants (the same literal keys as before, pinned by the existing g-key-002 test). objectlock_sys.rs evaluates retention/legal-hold/default-retention from them; the fail-closed error messages and decision logic are unchanged line for line where possible.

Boundary conversions:
- bucket/metadata_sys.rs gains default_retention_from_object_lock_config, converting the persisted s3s configuration into the storage-level DefaultRetention; a rule without a usable GOVERNANCE/COMPLIANCE mode converts to None exactly like the evaluation code always ignored it, and days/years pass through so an invalid period still fails closed at evaluation time.
- check_object_lock_for_deletion_with_config becomes check_object_lock_for_deletion_with_default_retention (it only ever read the default retention); the lifecycle object_lock_boundary keeps the old s3s-typed signature and converts.
- The ObjectLockApi / ObjectLockStatusExt trait impls for the s3s DTOs move next to the persisted configuration owner in bucket/metadata.rs; the traits stay in object_lock/mod.rs.
- check_retention_for_modification now takes Option<RetentionMode>. The serving-layer wrappers (rustfs storage_api, set_disk options path) convert the request string with the new RetentionMode::parse_exact, which accepts only the canonical spelling — preserving the historical literal comparison where a non-canonical requested mode reads as a mode change and stays blocked.
- rustfs app-layer wrappers return the storage types; the replication-overwrite gate in object_usecase.rs uses the typed API (legal_hold.is_on(), RetentionMode::Compliance).

Ratchet: the ecstore-scoped s3s counter drops 42 -> 39 and the repo-wide file counter 211 -> 208 in scripts/check_s3s_footprint.sh.

Verification: cargo check -p rustfs-ecstore --all-targets and -p rustfs (lib+bins); cargo clippy -p rustfs-ecstore --all-targets and -p rustfs --lib --bins (clean); cargo nextest run -p rustfs-ecstore --no-fail-fast (4534/4542; the 8 failures are the same store::rebalance / store::heal machine-baseline set that fails identically on pristine origin/main, plus one fencing flake that passes in isolation); all object_lock/retention/legal-hold tests pass; guard scripts (layer deps, migration rules, s3s footprint, logging, error-format ratchet, doc paths) pass.

crates/ecstore/src/api/mod.rs

@@ -154,6 +154,12 @@ pub mod bucket {
     pub mod object_lock {
         pub use crate::bucket::object_lock::{ObjectLockApi, ObjectLockStatusExt};
 
+        pub mod types {
+            pub use crate::bucket::object_lock::types::{
+                DefaultRetention, LegalHoldStatus, ObjectLegalHold, ObjectRetention, RetentionMode,
+            };
+        }
+
         pub mod objectlock {
             pub use crate::bucket::object_lock::objectlock::{get_object_legalhold_meta, get_object_retention_meta};
         }