Function enemy_power_for_chapter

pub fn enemy_power_for_chapter(chapter: i64) -> f64

Absolute enemy power at chapter: ANCHOR · ∏ step(c), where the per-chapter step is the constant chapter_power_step until enemy_step_taper_start, then decays smoothly toward enemy_step_late so late-game difficulty tracks the player’s decelerating power (no multi-day stalls). With the taper OFF (enemy_step_late >= chapter_power_step) this is the original closed form ANCHOR · STEP^(chapter − ANCHOR_CH). A tapering early-chapter bump (see [enemy_early_chapter_mult]) multiplies the result.

Note: the mid-game bump ([enemy_mid_chapter_mult]) is NOT applied here — it is applied only to CampaignBossFight in enemy_power_scalar, so wave fights and the global power curve remain unaffected (avoids monotonicity violations for large bump values).