[chrischeng-c4]

Closes #18

Summary of Changes

Implement the pure domain logic for BackupSet restore progress tracking as scoped in #18:

• Domain Model (RestoreLedger): Added RestoreLedger, ArtifactStage (Pending, Verified, Applied), RestorePhase (Staging, Applying, Committed), ResumePoint (Fresh, Continue, AlreadyCommitted), and ArtifactEntry under crates/service-backup/src/domain/restore_ledger.rs.
• Domain Errors (RestoreLedgerError): Added typed error enum RestoreLedgerError under crates/service-backup/src/domain/restore_ledger_error.rs covering invalid input, illegal artifact and phase transitions, unverified/unapplied sets, ConflictingSet conflicts carrying both IDs, and inconsistent deserialized phases.
• Exports: Declared and re-exported new types in crates/service-backup/src/domain.rs and crates/service-backup/src/lib.rs.
• Transitions & Serialization:
  • RestoreLedger::begin validates set ID, manifest digest format, and artifact bounds.
  • Legal transition methods mark_verified, begin_apply, mark_applied, and commit leave state unchanged on error and allow idempotent repeats.
  • resume and resume_or_begin identify matching vs conflicting sets.
  • Compact JSON serialization with "version": 1, 1 MiB limit, camelCase fields, and deny_unknown_fields.
• Test Suite:
  • Added new integration-test unit module crates/service-backup/tests/it/restore_ledger.rs registered in tests/it/main.rs.
  • Comprehensive coverage for: valid full path, all invalid begin inputs, illegal transitions, idempotent repeats, conflicting set / replaced manifest detection, already committed replay, JSON round-trips, rejected JSON documents, and crash-prefix model test verifying each artifact is applied exactly once.
  • Fixed needless .as_bytes().len() call in tests/it/projected_admin_transport.rs to satisfy clippy warnings.

Local Test Results

• cargo fmt -p service-backup -- --check: passed
• cargo clippy --locked -p service-backup --all-features --all-targets --no-deps -- -D warnings: passed (0 warnings)
• cargo check --locked -p service-backup --all-features --all-targets: passed
• cargo test --locked -p service-backup --test it restore_ledger: 9 passed, 0 failed, 0 ignored
• cargo test --locked -p service-backup --test it: 25 passed, 0 failed

Cloud CI Status (MR Gates)

• crate-gates (cargo test --locked -p service-backup --features http-client): PENDING cloud CI runner execution on PR head
• Integration/HTTP network tests: DEFERRED to authorized cloud CI per user test policy
• Release qualification acceptance matrix: NOT RUN (release operations not authorized)