Add BRC-157 entropy derivation helper

[rohenaz]

Implements the generic BRC-157 entropy-rooted wallet derivation helper in ToolboxWallet.

• validates all BIP-39 entropy sizes and the secp256k1 scalar range
• generates 32-byte entropy and 24-word mnemonics for new wallets
• derives profiles at hardened m/0-prime/i-prime without changing legacy MnemonicRestore
• preserves original mnemonic entropy length across BRC-140 share recovery
• pins the worked BRC vector and independently generated TypeScript profile vectors
• redacts secret material from diagnostic descriptions

Verification:

• focused BRC157Tests: 11 passed
• full Swift suite: 176 passed, 10 environment-gated live tests skipped
• strict formatting lint and diff checks passed

This is the reusable dependency for the separate unreleased 1Sat mobile cutover that will delete random profile pads rather than migrate them.

[rohenaz]

Superseded by opldotdev/swift-sdk#7 after independent boundary review. BRC-157 is a reusable composition of SDK-owned BIP-39, BIP-32, secp256k1, and BRC-140 primitives, so duplicating it in the concrete ToolboxWallet package would be the wrong dependency boundary. The SDK PR preserves the reviewed behavior/vectors and also aligns the underlying BRC-140 edge semantics.