lwwallet: reduce browser wallet public KDF cost

[Roasbeef]

Summary

In this PR, we reduce browser wallet create and unlock time by configuring
independent scrypt profiles for btcwallet's public and private master keys.
The fixed lwwallet public passphrase protects public wallet metadata, so its
creation profile uses N=16. The user-supplied private passphrase keeps
btcwallet's default N=262144 profile.

The change is limited to the WASM wallet bootstrap. Native builds retain their
existing loader path, existing browser databases retain their persisted KDF
parameters, and only newly created browser wallets receive the faster public
profile.

The latest five-run browser benchmark with
lightninglabs/wavelength-sdk#53 measured:

• Runtime ready p95: 276 ms.
• WASM load and instantiate p95: 138 ms.
• Wallet create total p95: 1,886 ms.
• Wallet unlock total p95: 669 ms.
• Performance budget violations: 0.

Technical Notes

This PR temporarily replaces github.com/btcsuite/btcwallet with the exact
fork commit from btcsuite/btcwallet#1298. The replace directive and fork
checksums can be removed once that PR merges and Wavelength advances to the
upstream commit.

The private KDF is selected explicitly rather than relying on an implicit
default. This keeps the security boundary visible in the call site.

Steps to Test

go test ./lwwallet
GOOS=js GOARCH=wasm go test -exec=true ./lwwallet
make lint-changed-local base=origin/main

For the browser benchmark:

WAVELENGTH_DIR=/path/to/wavelength \
  pnpm --dir /path/to/wavelength-sdk \
  --filter web-wallet-demo run wasm:local

pnpm --dir /path/to/wavelength-sdk perf:web

Related Issues and Pull Requests

• Depends on wallet: support independent create scrypt parameters btcsuite/btcwallet#1298.
• Supports web/core: speed up wasm load and wallet unlock, add perf instrumentation + benchmarking harness wavelength-sdk#53.

[gemini-code-assist]

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