channeld: document that local funding pubkey is fixed for splices

[Andezion]

Fixes #9500

splice_init and splice_ack always reuse channel->funding_pubkey[LOCAL] when building the replacement funding output. This is intentional - remote peer funding-key rotation during splices was added in dcdf7db, but the symmetric local-side feature (requesting a new key generation from the signer, for VLS/external-signer support and other stuff) was never built and isn't tracked anywhere in code or comments

This adds comments at the splice_init/splice_ack call sites and on struct splicing noting the current behavior is deliberate by omission, so this doesn't get mistaken for a bug in future review or signer-integration work. No behavior change!!

Changelog-None

Important

26.09 FREEZE August 5th: Non-bugfix PRs not ready by this date will wait for 26.12.

RC1 is scheduled on August 17th

The final release is scheduled for September 7th.

Checklist

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• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
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