br: support experimental full snapshot restore through RestoreRegion

## Feature Request

**Is your feature request related to a problem? Please describe:**

BR full snapshot restore currently drives the Store Download/MultiIngest pipeline. Restoring classic snapshot SSTs into a cloud storage engine target needs a caller for the Region-leader RestoreRegion RPC, so Workers can prepare the source SSTs and the Store can apply the output.

**Describe the feature you'd like:**

Provide an opt-in first-phase path for NextGen BR full snapshot restore from S3-compatible storage into newly created tables on a compatible CSE target. Reuse metadata/schema creation, key rewriting, Region splitting/scattering, progress and checksum infrastructure. Send source write/default SST descriptors and shared storage/cipher configuration to each Region leader, and report success only after the RPC completion.

A failed RPC may already have applied. This phase must not automatically replay successful Regions or resume an existing checkpoint. Retry/deduplication, resume, failure recovery and performance tuning require follow-up work.

**Describe alternatives you've considered:**

Keep the existing Download/MultiIngest path as the default. Wrapping RestoreRegion in the existing whole-file-group retry would risk repeating Regions that have already applied.

**Teachability, Documentation, Adoption, Migration Strategy:**

Expose a default-off `--experimental-restore-region` option. Reject unsupported restore modes, rate limiting and checkpoint resume with actionable errors. Include deterministic backup fixtures and raw-command/component-identity capture for manual validation.

Protocol dependency: https://github.com/pingcap/kvproto/pull/1533. This issue tracks the BR first phase; it does not claim production readiness or support for automatic retry.

No repository files or tests are named in the issue. Start by reading the existing Download/MultiIngest path and the linked kvproto RestoreRegion dependency, then trace how metadata, schema creation, Region splitting, progress, and checksums are handled. Done means a default-off option supports the specified first-phase restore, rejects unsupported modes and resume, and reports success only after RestoreRegion completes.

Written by the indexing model from the issue text.