Function activation_height_from_headers 

pub fn activation_height_from_headers<H>(
    headers: &[H],
    current_height: u64,
    current_time: u64,
    deployment: &Bip9Deployment,
) -> Option<u64>
where
    H: AsRef<BlockHeader>,

Computes the activation height for a BIP9 deployment from recent block headers.

• headers – Last N block headers (oldest first), typically the 2016 blocks before the block we are validating. Must be the period ending at current_height - 1.
• current_height – Height of the block we are validating.
• current_time – Network time (Unix timestamp) for start/timeout checks.
• deployment – BIP9 deployment (bit, start_time, timeout).

Returns Some(activation_height) when the last LOCK_IN_PERIOD headers (the retarget window ending at current_height - 1) show ≥ACTIVATION_THRESHOLD signalling for deployment.bit. Then activation_height = (period_index + 2) * 2016 where period_index = (current_height - 1) / 2016 (BIP9: ACTIVE at start of period period_index + 2).

This does not mean rules are active at current_height yet; use bip_validation::is_bip54_active_at(height, network, Some(activation_height)) for that.

When scanning the chain sequentially, merge multiple Some(h) values with h.min(...) so an earlier period’s lock-in (smaller activation height) is not overwritten by a later window’s larger computed height (see merge_bip54_activation_candidate).