feat(vc): add committer init-from-snapshot bootstrap mode

[sridhar-panigrahi]

⚠️ Stacked on top of #516. This PR's diff includes the StateImporter commit from that PR — please review #516 first or alongside this one. Once #516 merges, this PR will rebase onto main and the diff will collapse to only the bootstrap-mode files listed below.

What this PR adds

This is the missing seam between the Exporter CLI (hyperledger/fabric-x#97, hyperledger/fabric-x#180) and the StateImporter primitives in #516. It gives the committer a one-shot bootstrap mode so an operator can take a genesis-data file produced by the migration tool and use it to initialise a fresh Fabric-X state database.

fabric peer snapshot
      │
      │  fxmigrate export   (hyperledger/fabric-x#97)
      ▼
genesis-data file
      │
      │  committer init-from-snapshot   (this PR)
      │    └─► StateImporter             (#516)
      ▼
populated VC state database
      │
      │  fxmigrate verify   (hyperledger/fabric-x#180)
      ▼
integrity report

This addresses deliverable #3 of the LFX mentorship issue LF-Decentralized-Trust-Mentorships/mentorship-program#65 ("Bootstrap-from-snapshot feature in the Fabric-X committer").

Files

File	Purpose
service/vc/snapshot_loader.go	Streaming reader for the genesis-data binary format documented in hyperledger/fabric-x-rfcs#5. Defines SnapshotHeader and SnapshotData. SnapshotData implements the StateIterator interface from #516 so it plugs straight into ImportNamespaceState without an adapter layer.
service/vc/snapshot_loader_test.go	Eight unit tests covering the binary parser end-to-end: header round-trip, full entry stream, empty entries, binary value bytes, missing file, empty header, empty namespace, and a truncated entry to verify that a partial write surfaces as a non-nil Err() rather than a clean EOF.
service/vc/bootstrap.go	Orchestrator. BootstrapFromSnapshot opens the snapshot, creates a StateImporter, runs InitSchema → CreateNamespace → ImportNamespaceState → SetBlockHeight → VerifyImport, and cross-checks the imported row count against the header's declared count to catch a truncated snapshot before the operator starts the committer for real.
cmd/committer/init_cmd.go	The user-facing subcommand: committer init-from-snapshot --config <vc.yaml> --snapshot-data <genesis.bin>. Sibling to start and healthcheck. One-shot: imports the snapshot and exits.
cmd/committer/main.go	Registers the new subcommand on the root.

Operator workflow

# 1. Migrate state out of the source Fabric peer:
fxmigrate export \
  --snapshot ./peer/snapshots/completed/mychannel/100 \
  --channel  mychannel \
  --namespace token \
  --output   genesis.bin

# 2. Provision a fresh Fabric-X DB and bootstrap it:
committer init-from-snapshot \
  --config       vc-config.yaml \
  --snapshot-data genesis.bin

# 3. Cross-check before going live:
fxmigrate verify \
  --genesis  genesis.bin \
  --snapshot ./peer/snapshots/completed/mychannel/100

# 4. Start the committer normally — it will resume at block N+1:
committer start vc --config vc-config.yaml

Design choices worth flagging for the reviewer

• Independent format parser, not a cross-repo import. The genesis-data format is a contract documented in fabric-x-rfcs#5; the producer (fxmigrate, in hyperledger/fabric-x) and the consumer (this committer) implement it independently. Avoids a Go module dependency between fabric-x-committer and fabric-x flowing in either direction. Open to revisiting if you'd prefer one canonical implementation.
• Subcommand sibling, not a flag on start. Bootstrap is one-shot initialisation, not a service mode. Matches the structure used by healthcheck and avoids overloading start vc with semantics that change behaviour mid-flight.
• Single namespace per snapshot file. That is what the genesis-data format already supports; multi-namespace would be a format change and should be addressed in the RFC first.
• No idempotency / re-run guard. CreateNamespace fails if the target namespace already exists; surfacing the conflict to the operator is preferable to silently merging into existing state on a re-run.
• No transaction-ID history migration. Consistent with the RFC's decision (security/format incompatibility); only state and block height are bootstrapped.

What's deliberately deferred

• End-to-end integration tests against a real Postgres — both the bootstrap orchestrator and the CLI surface tests need a running DB. I'd like to add them on this branch in a follow-up commit once the design is acked, rather than bloat the initial review.
• Policy import. The current genesis-data format from fxmigrate doesn't carry namespace policies; once the format is extended, BootstrapFromSnapshot can call ImportPolicy from this same orchestrator with no change to the CLI surface.

Testing

• go build ./... — clean.
• go vet ./... — clean.
• golangci-lint run --new-from-rev=upstream/main — no findings on the files added by this PR.
• go test ./service/vc/... -run TestOpenSnapshotData — all eight unit tests pass.

Related work

• RFC: hyperledger/fabric-x-rfcs#5
• StateImporter primitives: feat(vc): add StateImporter for bulk state initialization from external sources #516 (this PR depends on it)
• Migration tool — Exporter CLI: hyperledger/fabric-x#97
• Migration tool — verify command: hyperledger/fabric-x#180
• LFX mentorship issue: LF-Decentralized-Trust-Mentorships/mentorship-program#65