feat: prorate early exit penalty

[dangelo352]

Summary

• Implements pro-rata early-exit penalties for commitment_core based on remaining commitment duration.
• Keeps penalties in CollectedFees(asset) for FeeRecipient withdrawal, matching the existing fee architecture.
• Adds stable early_exit_settled event payload with amount, penalty, and elapsed_ratio_bps while preserving the legacy EarlyExt event.
• Documents the formula and fee routing in docs/EARLY_EXIT.md and docs/FEES.md.

Fixes #464.

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-prorata-exit-target cargo test -p commitment_core prorata -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-prorata-exit-target cargo test -p commitment_core fee -- --nocapture