feat(signal-registry): implement proportional fee splitting for collaborative signals

[syed-ghufran-hassan]

• Add PendingRewards storage struct and StorageKey variant to track pending fee/ROI claims per address.
• Introduce internal helpers (get_pending_rewards, store_pending_rewards, add_pending_rewards) for managing pending rewards.
• Add public claim_pending_rewards function for users to withdraw accumulated rewards (pull model).
• Modify record_signal_outcome to accept total_fee and total_roi; when the signal is collaborative, compute per‑author distributions using distribute_collaborative_rewards, emit CollaborativeRewardDistributed event, and credit pending rewards for each author.
• For non‑collaborative signals, continue to credit all fees to the primary provider.
• Add necessary imports for collaboration module functions.

Closes # #783