refactor(common): move scanner/heal contracts into dedicated crates (#6615)

refactor(common): move scanner/heal domain contracts into dedicated crates

crates/common carried ~5.6K lines of scanner/heal domain code (metrics.rs,
heal_channel.rs, last_minute.rs) parked there to break dependency cycles;
every scanner type change recompiled all 11 rustfs-common dependents.

Pure move, zero renames, zero shape changes (backlog#1843):

- New crate rustfs-heal-contracts receives heal_channel.
- New crate rustfs-scanner-contracts receives metrics, last_minute, and the
  GLOBAL_INIT_TIME trio (metrics::report() reads it as the current-cycle
  fallback, so it must live below the shim to avoid a dependency cycle).
- rustfs-common re-exports everything at the old paths as a transitional
  shim; consumers migrate crate by crate, then the shims are deleted.

crates/scanner-contracts/src/lib.rs

@@ -0,0 +1,19 @@
+// Copyright 2024 RustFS Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+pub mod init_time;
+pub mod last_minute;
+pub mod metrics;
+
+pub use init_time::{GLOBAL_INIT_TIME, get_global_init_time, set_global_init_time_now};