feat(packages): multi-provider pegin support in PeginManager

[jonybur]

Parent epic: #1633

Summary

Generalize PeginManager.preparePegin to accept multiple vault providers in one call. Today it accepts vaultProviderBtcPubkey: string (singular); needs to accept an array of providers and produce per-provider HTLC outputs while keeping the shared Pre-PegIn structure intact.

This is the foundation for the entire batch-vault-creation initiative — every other ticket in #1633 depends on this landing.

Scope

• Extend PreparePeginParams with vaultProviders: { address: string; btcPubkey: string }[] (deprecate the singular field; keep a single-element-array shim for one release).
• Update preparePegin() to loop sizing + commitment construction per provider while keeping the shared Pre-PegIn output structure intact.
• Return: { perProvider: { provider: Address; transaction: PreparePeginTransaction }[] }.
• Per CLAUDE.md §4: vault-secret derivation is per-vault, so each provider's vault gets its own root derivation. Add a two-vault test across distinct providers with overlapping funding inputs (mandatory per §4).

Files

• packages/babylon-ts-sdk/src/tbv/core/managers/PeginManager.ts
• packages/babylon-ts-sdk/src/tbv/core/primitives/psbt/pegin.ts (only if build helpers need adjusting)

Acceptance

• Unit test: single-provider call produces output byte-identical to the current single-provider flow.
• Unit test: two providers with overlapping funding inputs produce distinct vault roots (CLAUDE.md §4 two-vault test requirement).
• All existing pegin tests pass unchanged.
• Golden vectors in this repo and btc-vault re-verified.

Dependencies

None — this is foundational.

[jrwbabylonlab]

Closing as not planned. The multi-provider PeginManager API described here was never built and has been superseded by single-provider UTXO-split batching (#1783/#1823). Reopen if multi-provider deposits are revived.