v0.1.0-beta.2 — re-cut against merged feat/external-signer (darwin-arm64 only)

Bytecode-equivalent re-cut of v0.1.0-beta.1 against the merged tip of
UTEXO-Protocol/rgb-lightning-node:feat/external-signer (commit 53655f7,
which includes PR #40).

No functional changes vs beta.1. Build provenance now points at the
upstream merge commit rather than the Jainakin fork branch.

Prebuild: darwin-arm64 only. Other platforms still pending CI.