Replace the proposer indices cache usage in data column sidecar
verification with direct state lookahead access. Since data column
sidecars require the Fulu fork, the state always has a ProposerLookahead
field that provides O(1) proposer index lookups for current and next
epoch.
This simplifies SidecarProposerExpected() by removing:
- Checkpoint-based proposer cache lookup
- Singleflight wrapper (not needed for O(1) access)
- Target root computation for cache keys
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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
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