maintainer: make compareEventKey return 0 for equal event keys

Issue

compareEventKey currently returns 1 when two eventKey values are equal.

Current code in maintainer/barrier_helper.go:

func compareEventKey(a, b eventKey) int {
	if a.blockTs < b.blockTs {
		return -1
	}
	if a.blockTs > b.blockTs {
		return 1
	}
	if !a.isSyncPoint && b.isSyncPoint {
		return -1
	}
	return 1
}

For equal blockTs and equal isSyncPoint, the comparator should return 0.
Returning 1 violates the normal comparator contract used by slices.SortFunc and can also make future heap/sort usages harder to reason about.

Suggested fix

Handle both tie-break directions explicitly, then return 0 for equality:

func compareEventKey(a, b eventKey) int {
	if a.blockTs < b.blockTs {
		return -1
	}
	if a.blockTs > b.blockTs {
		return 1
	}
	if !a.isSyncPoint && b.isSyncPoint {
		return -1
	}
	if a.isSyncPoint && !b.isSyncPoint {
		return 1
	}
	return 0
}

Start in maintainer/barrier_helper.go at compareEventKey and inspect its slices.SortFunc usage. Verify the comparator's ordering for blockTs and isSyncPoint combinations, then run the relevant maintainer tests. Done means equal event keys compare as 0 while the existing ordering remains unchanged.