Function isBranchEligible

A branch is eligible only inside the rollback horizon: currentTipEpoch - forkEpoch <= maxRewindCommits (convergence.md "Eligibility").

This predicate is intentionally horizon-only and takes no retained anchor. convergence.md lists a second eligibility rule — "a branch that needs a retained state older than the retained anchor MUST NOT be selected" — but that guard is deliberately kept separate from this predicate, mirroring the reference engine's two-layer structure (is_branch_eligible is horizon-only; the anchor check lives in candidate admission + late-commit classification, always anchor-first). Keeping the anchor out of here lets the predicate be reused for horizon-only purposes (e.g. eligibility counts) and keeps it a pure function of policy + branch, with no dependency on retained-store state. The anchor guard is applied operationally: classifyLateCommit emits beyond_anchor for a sub-anchor commit (ingest), and fork recovery cannot build a branch whose forkEpoch state is no longer retained.