fix: default message padding to false

[vincenzopalazzo]

Summary

• Default --message-padding to false.
• Keep padding available as an opt-in with --message-padding=true.
• Update docs/help/changelog to match the compatibility default.

Rationale

• CLN padding uses BOLT-valid no-reply pings.
• Deployed LND versions before lightningnetwork/lnd@08b26b6 reject those pings with pong bytes exceeded.
• Defaulting off avoids disconnects/channel-open failures while preserving the feature for operators who opt in.

Mitigates #9108

Test plan

• git diff --check
• make lightningd/lightningd.o (fails locally because the worktree is not configured: config.vars is missing and ./configure has not been run)

[michael1011]

Concept ack. Message padding also breaks interoperability with the latest Eclair release v0.13.1

[daywalker90]

Concept ack. Message padding also breaks interoperability with the latest Eclair release v0.13.1

it shouldn't: #9022

[vincenzopalazzo]

@daywalker90, you're kind of wrong because the last release of Eclair is https://github.com/ACINQ/eclair/releases/tag/v0.13.1, which is pre-patch.

We're not saying there's no patch — we're saying that not all the patch releases have been released yet. Setting this flag to true will only increase the frustration of users who can no longer use their setups.

For example, all Ocean miners running CLN have a single channel with Eclair or Megalithic.me, so both versions are currently broken and the result will be that people will migrate away from cln because at the current state there are better solution out there.

This kind of thing surprises me because the bug has been in mainline since 24.02, and in all this time no one has caught or complained about it. This means people are quite a bit behind the mainline version of the implementation, and it could be a while before they start running the patched versions.

p.s: Probably this patch should also be backported to v24.02