Function select_n3_tier 

pub fn select_n3_tier(pcid: u16) -> N3Tier

Select the N3 tier based on actual PCID capability.

Distinguishes theoretical CPU support from operational capability:

• N3Tier::PcidPreserving requires PCID available AND a valid PCID > 0
• N3Tier::FullIsolation is used when PCID is unavailable or exhausted