[Feature]: foc2foc payer-migrate — re-commit a data set's pieces to a new data set and payer

**Description:** A data set's payer is fixed at creation. Verified against the deployed contracts: in Filecoin Pay v1 a rail's `from` is written only by `createRail`, and in FilecoinWarmStorageService 1.3.0 `info.payer` is written only in the data-set-creation path (`storageProviderChanged` reverts; Pay v1 is non-upgradeable). Neither contract exposes a re-point function. So moving already-stored data under a new payer means creating a new data set and re-committing its pieces.

Those pieces are re-pullable without the original source. A storage provider serves each stored piece's canonical CAR at `GET /piece/{pieceCidV2}` (verified live against a mainnet SP — HTTP 200, `etag` equals the pieceCID, CARv1 body), and PieceCIDs are deterministic, so a re-pull recomputes the same commitment. The existing on-chain data is therefore its own migration source — no gateway or original CID list required.

**Proposed flow (foc2foc / payer-migrate):**
1. Enumerate a source data set's active pieces on chain (`getActivePieces` → pieceCID + rawSize) and seed a plan/DB from them.
2. Create the destination data set under the new payer.
3. Submit with the PDP pull source pointed at the existing SP's `/piece/{pieceCidV2}`. `pdp-submit --source-base https://` already builds `{base}/piece/{pcid}` (see `pullSourceUrl` in `packages/cli/src/submit-pdp.ts`); the only new work is seeding the plan from chain rather than from a CID list.

**Scope:**
- New payer as a different **EOA** works with existing `create-data-set` (payer = the signer).
- A **contract/sponsor payer** is out of scope here: the contract must `setOperatorApproval` and authorize a session key before creation. Track separately.

**Caveats:** the source SP must keep serving `/piece` through the migration, and storage plus a payment rail are duplicated until the old data set is terminated. Related: #49 (relocate a single failed copy to another provider) is the same "move a copy" theme at piece granularity.