ldk: how to best convert a channel from cln to ldk

(@vincenzopalazzo suggested to leave a comment so he can recall some stuff later https://discord.com/channels/@me/1244713070765539481/1276450545351000074)
It's important to understand how these values are used in core lightning, how they are set in graph, so we can know what we're doing is correct.
https://github.com/ElementsProject/lightning/blob/master/common/gossmap.c#L525

_Originally posted by @raehat in https://github.com/tareknaser/barq/pull/29#discussion_r1730564969_

Start with the discussion in PR #29 and inspect Core Lightning's common/gossmap.c around line 525. Determine how the channel values are set and used in Core Lightning's graph, then compare that behavior with the channel conversion from CLN to LDK. Done means the correct mapping and its rationale are established.

Written by the indexing model from the issue text.