Break claim_winnings payout into named steps

Split the single pro-rata expression into intermediary variables
(losing_pool_share_numerator, winnings, payout) so each step of the parimutuel
split reads on its own line.

Author: claude

tokens/betting-market/anchor/programs/betting-market/src/instructions/claim_winnings.rs

@@ -59,15 +59,19 @@ pub fn handle_claim_winnings(context: Context<ClaimWinnings>) -> Result<()> {
     );
 
     let stake = context.accounts.bet.amount;
+    // Total staked on the winning outcome, and the losers' stakes after the fee.
     let winning_pool = context.accounts.event.winning_pool;
     let distributable_losing_pool = context.accounts.event.distributable_losing_pool;
 
-    // Pro-rata share of the losing pool, on top of the returned stake. u128
-    // intermediate avoids overflow; the floor leaves at most a few base units
-    // of dust in the vault.
-    let winnings_share =
-        (stake as u128 * distributable_losing_pool as u128 / winning_pool as u128) as u64;
-    let payout = stake + winnings_share;
+    // Parimutuel split: winners share the losing pool in proportion to their
+    // own stake. Work in u128 and divide once, after the multiply, so the
+    // result is floored a single time — dividing first would throw away
+    // precision. The floor leaves at most a few base units of dust in the vault.
+    let losing_pool_share_numerator = stake as u128 * distributable_losing_pool as u128;
+    let winnings = (losing_pool_share_numerator / winning_pool as u128) as u64;
+
+    // Winners always get their own stake back on top of their winnings.
+    let payout = stake + winnings;
 
     let event_id = context.accounts.event.event_id;
     let event_bump = context.accounts.event.bump;