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-examined the full refactored persistence loop against the original control flow. Verified:

• needs_cm_persist flag-gating + eager CM poll (lines 1123-1162): pending_monitor_writes is captured inside the async body during the eager poll, which runs after get_and_clear_needs_persistence() — race window preserved.
• Synchronous side effects (network graph prune at 1212, have_pruned/time_passed/scorer stepping) all run before futures are constructed, matching original ordering.
• break semantics on scorer/sweeper timers (lines 1255, 1264) and check order preserved.
• Skipped-work futures resolve Ok(()), matching the old unset-slot semantics.
• Pin-moves into Joiner after eager poll are sound.

No new issues found beyond what was raised in the prior review pass (the scorer_fut formatting note at lib.rs:1284-1285, already noted, remains non-blocking).

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.

[TheBlueMatt]

This can stay in the if/future and avoid the if.

[TheBlueMatt]

I don't believe there's any reason to change this, it would probably be simpler to keep the code structured the way it was.

[Abeeujah]

I've simplified it, although, I couldn't fully restore the original now that the futures are stack-pinned, Joiner holds Pin<&mut> references into them and so must be declared after all of them.

Meaning it doesn't exist yet at the eager-poll site, so the outcome has to be stashed and fed in once the Joiner is constructed. Open to a cleaner approach if you have any recommendations.

[codecov]

Codecov Report

❌ Patch coverage is 86.76471% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.25%. Comparing base (b6f3ad7) to head (583404e).
⚠️ Report is 3215 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning-background-processor/src/lib.rs	86.76%	7 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4733      +/-   ##
==========================================
+ Coverage   84.54%   86.25%   +1.70%     
==========================================
  Files         137      160      +23     
  Lines       77617   111579   +33962     
  Branches    77617   111579   +33962     
==========================================
+ Hits        65625    96241   +30616     
- Misses       9949    12694    +2745     
- Partials     2043     2644     +601     

Flag	Coverage Δ
tests	86.25% <86.76%> (?)

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