Avoid heap-allocating background processor futures

[Abeeujah]

The persistence loop in process_events_async boxed each task future
(ChannelManager, network graph, scorer, sweeper, LiquidityManager)
before handing it to the Joiner, incurring a heap allocation per task
on every iteration. Box::pin was only needed to satisfy the Joiner's
Unpin bound when our MSRV predated core::pin::pin!.

Now that the MSRV is 1.75, replace Box::pin with core::pin::pin! so the
futures live on the stack. Pin<&mut F> is Unpin, so the Joiner bounds
still hold. To pin every future before the Joiner borrows it, build all
five uniformly at the end of the iteration: run the synchronous timer
checks and side effects first, capture their outcomes in flags, and
gate each future's work behind those flags. Futures whose work is
skipped resolve to Ok(()) immediately, preserving the previous behavior
where an unset Joiner slot was treated as Ready(Ok(())).

The eager single poll of the ChannelManager future and the early
loop-exit (break) semantics on the scorer/sweeper timers are unchanged.

[ldk-reviews-bot]

I've assigned @valentinewallace as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[ldk-claude-review-bot]

I've re-verified the full refactored block against the original behavior. The code matches my prior analysis: side-effect ordering, skipped-work semantics, eager CM poll, break semantics, and drop order are all preserved.

No new issues found beyond what I noted previously.

The scorer_fut formatting (lib.rs:1294-1295 — the let scorer_fut = split before core::pin::pin!) is still inconsistent with the sibling network_graph_fut/sweeper_fut bindings and would likely be reformatted by cargo fmt; this was already raised in the prior pass and remains a non-blocking style note, not a new finding.

No issues found.

[ldk-reviews-bot]

🔔 1st Reminder

Hey @valentinewallace! This PR has been waiting for your review.
Please take a look when you have a chance. If you're unable to review, please let us know so we can find another reviewer.