chore: index proposer preferences by slot first

[markolazic01]

Motivation

As suggested by @twoeths in #9303, the current map nesting in SeenProposerPreferences (Map<dependentRoot, Map<slot, validatorIndex>>) makes pruning inefficient — it requires iterating through every dependent root and checking each slot individually.

Description

Swaps the map nesting to Map<slot, Map<dependentRoot, validatorIndex>> so that pruning can delete entire slot entries directly from the outer map. No changes to the public API; isKnown, add, and prune all retain their existing signatures and behavior.

Closes #9309

AI Assistance Disclosure

• External Contributors: I have read the contributor guidelines and disclosed my usage of AI below.

Used Claude to discuss the approach and review the implementation.

[gemini-code-assist]

Code Review

This pull request refactors the SeenProposerPreferences class by reordering its internal map structure to use Slot as the primary key instead of RootHex. This change simplifies the prune method, allowing it to iterate directly over slots and remove outdated entries in a single pass. A review comment suggests optimizing the prune method further by using .keys() instead of .entries() to reduce memory overhead during iteration.

[nflaig]

lgtm, thanks @markolazic01 for fixing this

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.59%. Comparing base (87cbe69) to head (fa68f3a).
⚠️ Report is 2 commits behind head on unstable.

Additional details and impacted files

@@            Coverage Diff            @@
##           unstable    #9312   +/-   ##
=========================================
  Coverage     52.59%   52.59%           
=========================================
  Files           848      848           
  Lines         61087    61087           
  Branches       4504     4504           
=========================================
  Hits          32126    32126           
  Misses        28897    28897           
  Partials         64       64           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[wemeetagain]

🎉 This PR is included in v1.43.0 🎉