feat: add Fabric snapshot to Fabric X migration RFC

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Description

This RFC defines an offline migration from a Fabric Classic peer snapshot to
Fabric-X.

It specifies:

• validation of the source snapshot and deterministic genesis-data bundle;
• one Fabric channel per Fabric-X network, with mapped application namespaces;
• migration of public state and transaction IDs for anti-replay;
• explicit recording of PDC exclusions (hashes) without exposing private values or
  hashes as public state;
• atomic committer bootstrap at target anchor B; and
• verification, idempotency, activation, multi-organization agreement, and
  recovery.

Source block history, online migration, PDC values, automatic chaincode
translation, and channel consolidation are outside this proposal.

Proof of concept

• Exporter and genesis-data bundle
• Fabric-X committer implementation

Related issues

• Fabric to Fabric-X ledger migration mentorship project

cc @liran-funaro @cendhu @tock-ibm

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PoC testing coverage

The migration integration test uses the exporter and disposable Fabric and Fabric-X networks. The primary path:

1. creates the Fabric-X network and target namespace;
2. records and freezes the target at anchor B;
3. exports and imports the genesis-data file;
4. verifies the imported state, transaction IDs, target configuration, namespace map, installed policy, and migration record;
5. activates the migration and restarts Fabric-X services; and
6. submits a normal transaction through the orderer and confirms that it updates an imported key from version 0.

The integration scenarios cover:

• Fabric 2.5.16 and Fabric 3.1.5 LevelDB snapshots;
• a Fabric 3.1.5 CouchDB snapshot;
• retrying the same bundle without importing its rows again;
• rejecting imports after activation;
• two source channels migrated into separate Fabric-X networks;
• two committer organizations importing the same bundle, reporting identical verification results, and committing the same later transaction; and
• reconstructing an empty committer database by processing Fabric-X blocks through B, reapplying the same bundle, and then processing the later Fabric-X blocks.

The rebuild scenario reuses the original bundle. It does not test whether a replacement bundle is rejected after the database-local migration record has been lost.

The committer bootstrap tests cover the failure paths separately: atomic rollback, missing or non-empty namespaces, existing transaction IDs, missing block 0, unsupported target policies, changed state or policy bindings, changed anchors, and attempts to use a different bundle while the migration record exists.

Run the focused tests with:

go test ./cmd/committer ./service/vc
go test -tags=integration ./integration/migration \
  -run TestInitFromSnapshot \
  -count=1 -v

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On a second note, not needed since they'll be rejected. To be removed from the snapshot and import.
Extra point - have an integration test verify this.