Use inclusive ramp-start boundary for initial pledge ramp parameters

[Copilot]

Forest’s initial pledge ramp-start handling differed at the boundary epoch, which could skip ramp parameters when current_epoch == ramp_start_epoch. This aligns the boundary semantics with the intended inclusive behavior used by Lotus.

• Ramp-start boundary logic

  • Updated initial pledge ramp parameter selection to treat ramp start as active at the boundary epoch (start <= current), while preserving the existing start > 0 gate.

• Single-purpose ramp helper

  • Extracted ramp parameter computation into a small helper used by compute_initial_pledge_for_power, making the boundary rule explicit and centralized.

• Focused boundary regression coverage

  • Added targeted test cases for:
    • ramp disabled (start == 0)
    • pre-start (current < start)
    • boundary (current == start)
    • post-start (current > start)

fn pledge_ramp_params(
    current_epoch: ChainEpoch,
    ramp_start_epoch: ChainEpoch,
    ramp_duration_epochs: u64,
) -> (ChainEpoch, u64) {
    if ramp_start_epoch > 0 && ramp_start_epoch <= current_epoch {
        (current_epoch - ramp_start_epoch, ramp_duration_epochs)
    } else {
        (0, 0)
    }
}

• Fixes Use inclusive ramp-start condition in initial pledge calculation #7501

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