Apply credibility ramp linearly, not cubed

[LandynDev]

Problem

The credibility ramp was being cubed along with the raw success rate in the emission formula. success_rate() returned the product raw_rate × ramp, and calculate_miner_rewards then raised that whole value to SUCCESS_EXPONENT (3) — so the effective term was raw_rate³ × ramp³ instead of raw_rate³ × ramp.

Impact (perfect raw rate, 6/10 closed swaps → ramp 0.6):

• Before: (1.0 × 0.6)³ = 0.216 → 21.6% of pool
• After: 1.0³ × 0.6 = 0.6 → 60% of pool

Worst for new miners: 1/1 completed earned 0.1³ = 0.001 (≈0) instead of 0.1. The intended linear on-ramp had become a near-cliff.

Fix

• success_rate() now returns the raw completed / closed ratio (cubed in the reward).
• New credibility_ramp() returns the linear ramp multiplier, applied once, uncubed.
• Reward: pool × crown_share × raw_rate³ × ramp × capacity × volume_factor.

Tests

• Split TestSuccessRateHelper into raw-rate and ramp helper tests.
• Updated end-to-end ramp expectations (1/1 → 0.1×pool, 5/5 → 0.5×pool, recycle remainder). Full-ramp and timeout cases are numerically unchanged.
• Full suite: 528 passed.