Function select_candidates 

pub fn select_candidates(
    config: &TierConfig,
    now: u64,
    blocks: &[(BlockKey, &BlockMeta)],
) -> Vec<MigrationCandidate>

Select blocks that need tier migration.

Returns candidates sorted by priority:

• Upgrades first, ordered by highest score (hottest blocks promoted first).
• Then downgrades, ordered by lowest score (coldest blocks demoted first).