Fix stock LDK public channel announcements

[bitwalt]

Fixes #12.

Root cause

RGB-LDK serialized the optional RGB contract ID into every BOLT 7 channel announcement, including an explicit absent marker for vanilla channels. Stock LDK signs the standard message without that byte, so RLN verified its peer signature against different bytes and force-closed the channel.

What changed

• Keep vanilla channel announcements byte-identical to BOLT 7 by omitting the RGB field when no contract is present.
• Accept standard stock-LDK announcements that end after the funding keys.
• Preserve support for the existing RGB extension and legacy absent marker.
• Pin RLN to the updated rust-lightning interoperability branch.
• Extend the stock-LDK fixture with announced-channel support and an end-to-end regression that mines through the announcement exchange, verifies the channel remains public/ready/usable, pays both ways, and closes cooperatively.

Impact

Announced vanilla channels with stock-LDK peers can complete the announcement-signature exchange without a destructive force-close.

Validation

• cargo check -p lightning --features electrum
• cargo fmt --package rgb-lightning-node -- --check
• cargo fmt --manifest-path src/test/interoperability/ldk-node/Cargo.toml -- --check
• cargo check --manifest-path src/test/interoperability/ldk-node/Cargo.toml
• cargo check --all-targets
• git diff --check

The focused rust-lightning unit-test target is currently blocked by unrelated pre-existing test-build failures in the fork; the production library and the full RLN target graph compile successfully. The announced-channel regression runs in the dedicated interoperability workflow.