wallet: support independent create scrypt parameters

[Roasbeef]

Change Description

In this PR, we let wallet creators select independent scrypt parameters for
the public and private master keys. The motivating case is a wallet with a
known public passphrase and a user-supplied private passphrase: callers can
reduce the public-key setup cost without weakening private-key encryption.

Existing Create entry points keep their old behavior by applying the same
config, or the same default, to both keys. The loader copies supplied configs,
rejects changes after a wallet is loaded, and does not alter existing wallet
databases. A regression test also uses identical public and private
passphrase bytes and verifies that their persisted KDF profiles remain
independent.

This supports the Wavelength browser-wallet optimization tracked in
lightninglabs/wavelength-sdk#53. With public N=16 and the private key left at
the default N=2^18, the downstream prototype reduced create p95 from 2,735
ms to 1,782 ms and unlock p95 from 1,220 ms to 662 ms.

Steps to Test

go test ./waddrmgr ./wallet

The full go test ./... run passes the changed packages. Its
chain/TestBitcoindEvents integration test fails locally because the spawned
Bitcoin Core v30 process does not remain available. The same failure
reproduces on the untouched origin/master base.

Pull Request Checklist

Testing

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• Tests covering the positive and negative (error paths) are included.
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