fix: address AI review feedback for #317 event replay

- Cache pool.tickSpacing() as immutable in EventReplayer constructor
  to avoid a repeated external call per _replayMint() invocation
- Rename driftCount → driftCheckpoints for consistency with log label
- Add sqrtDriftBps to the per-checkpoint progress log line, using the
  now-live lastExpectedSqrtPrice field (previously written but never read)
- Guard _replaySwap(): skip and count events where amountSpecified ≤ 0,
  which would silently flip exact-input into exact-output mode
- Add a final drift sample after the while-loop for trailing events not
  covered by the last LOG_INTERVAL checkpoint
- Move EventReplayer construction outside the broadcast block in
  BacktestRunner (it uses vm.* cheat codes incompatible with real RPC)
- Change second vm.closeFile() from try/catch to a direct call so errors
  surface rather than being silently swallowed

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

onchain/script/backtesting/BacktestRunner.s.sol

@@ -88,11 +88,13 @@ contract BacktestRunner is Script {
         // Deploy factory + pool and initialise at the resolved price.
         ShadowPool memory sp = ShadowPoolDeployer.deploy(address(tokenA), address(tokenB), sqrtPriceX96);
 
-        // Deploy the event replayer with the canonical token ordering from the pool.
-        EventReplayer replayer = new EventReplayer(sp.pool, MockToken(sp.token0), MockToken(sp.token1));
-
         vm.stopBroadcast();
 
+        // Instantiate EventReplayer outside the broadcast block: it uses Foundry cheat codes
+        // (vm.readLine, vm.roll, vm.warp) that only work in the forge simulation context and
+        // must not be sent as real transactions to the RPC endpoint.
+        EventReplayer replayer = new EventReplayer(sp.pool, MockToken(sp.token0), MockToken(sp.token1));
+
         // Query pool state (view calls, no broadcast needed).
         (uint160 slot0SqrtPrice, int24 tick,,,,,) = sp.pool.slot0();
         uint128 liquidity = sp.pool.liquidity();
@@ -129,8 +131,8 @@ contract BacktestRunner is Script {
                         l = vm.readLine(eventsFile);
                     }
                 }
-                // Reset again before the replay pass.
-                try vm.closeFile(eventsFile) {} catch {}
+                // Reset before the replay pass; we know the file is open after the count loop.
+                vm.closeFile(eventsFile);
 
                 console2.log("\n=== Starting Event Replay ===");
                 console2.log("Events file:    ", eventsFile);