fix(amm): address Copilot review — exact BigInt min_out, fail on oversized pool fee, sync flake run docs

apps/amm/flake.nix

@@ -18,8 +18,10 @@
   # fails flake evaluation because the app directory is copied into the Nix
   # store as its own flake root, so `../..` can't escape it there. Instead,
   # the repo-root flake.nix builds this module directly (src = ./apps/amm)
-  # and resolves amm_client_ffi via `self`. Build/run the app from the repo
+  # and resolves amm_client_ffi via `self`. The repo-root flake exposes the UI
-  # root, e.g. `nix build .#packages.<system>.default` or `nix run .`.
+  # as a named attribute (there is no bare `default`): run it from the repo root
+  # with `nix run .#amm-ui`, and build just the FFI crate with
+  # `nix build .#amm_client_ffi`.
   outputs = inputs@{ logos-module-builder, ... }:
     logos-module-builder.lib.mkLogosQmlModule {
       src = ./.;

apps/amm/qml/components/swap/SwapCard.qml

@@ -239,7 +239,15 @@ Rectangle {
         root.swapInProgress = true
         root.swapError = ""
 
-        var minOutStr = root.formatBaseUnits(root.minReceivedAmount)
+        // Compute the submitted slippage floor with exact integer (BigInt) math
+        // rather than the double-based preview: base-unit values for 18-decimal
+        // tokens exceed 2^53, where doubles would understate min_out and weaken
+        // price protection. Sell/buy reserves follow the pool's canonical order.
+        var minOutStr = swapState.minOutBaseUnits(
+                root.sellInput,
+                root.sellIsPoolA ? root.poolReserveA : root.poolReserveB,
+                root.sellIsPoolA ? root.poolReserveB : root.poolReserveA,
+                root.slippageTolerancePercent)
         // Max u64 sentinel: "ignore deadline", per AmmUiBackend.rep.
         var deadline = "18446744073709551615"
 

apps/amm/qml/state/DummySwapState.qml

@@ -73,6 +73,44 @@ QtObject {
         return safeAmount * (1 - safeSlippage / 100);
     }
 
+    // Exact-integer minimum-received (base units) for a SwapExactInput, used as
+    // the on-chain slippage floor that is actually submitted. Computed in BigInt
+    // (arbitrary precision, mirroring the on-chain u256 math): base units for
+    // 18-decimal tokens exceed 2^53 and even overflow u128 intermediates, so JS
+    // doubles silently lose precision and would understate min_out — weakening
+    // the user's price protection. amountIn/reserveIn/reserveOut are base-unit
+    // integer strings; returns a decimal string. Falls back to the double
+    // estimate only if BigInt is unavailable in this Qt build.
+    function minOutBaseUnits(amountIn, reserveIn, reserveOut, slippagePercent) {
+        if (typeof BigInt !== "undefined") {
+            var toBig = function (x) {
+                var s = String(x).trim();
+                return /^[0-9]+$/.test(s) ? BigInt(s) : BigInt(0);
+            };
+            var zero = BigInt(0);
+            var denom = BigInt(10000);
+            var amtIn = toBig(amountIn);
+            var resIn = toBig(reserveIn);
+            var resOut = toBig(reserveOut);
+            if (amtIn <= zero || resIn <= zero || resOut <= zero)
+                return "0";
+
+            var feeBps = BigInt(Math.round(Math.min(10000, Math.max(0, Number(root.feeBps) || 0))));
+            var amtInAfterFee = amtIn * (denom - feeBps) / denom;      // floor
+            if (amtInAfterFee <= zero)
+                return "0";
+            var out = resOut * amtInAfterFee / (resIn + amtInAfterFee); // floor
+            var slipBps = BigInt(Math.round(clampSlippagePercent(slippagePercent) * 100));
+            if (slipBps < zero) slipBps = zero;
+            if (slipBps > denom) slipBps = denom;
+            var minOut = out * (denom - slipBps) / denom;              // floor
+            return minOut.toString();
+        }
+        // Legacy double fallback (no worse than before if BigInt is missing).
+        var estOut = amountOutFor(amountIn, reserveIn, reserveOut);
+        return String(Math.floor(Math.max(0, minReceived(estOut, slippagePercent))));
+    }
+
     function maxSent(amountIn, slippagePercent) {
         const safeAmount = parseAmount(amountIn);
         const safeSlippage = clampSlippagePercent(slippagePercent);

programs/amm/client-ffi/src/pool.rs

@@ -27,7 +27,8 @@ pub fn decode_pool(bytes: &[u8]) -> Result<PoolView, String> {
         reserve_a: p.reserve_a,
         reserve_b: p.reserve_b,
         liquidity_supply: p.liquidity_pool_supply,
-        fees: u32::try_from(p.fees).unwrap_or(u32::MAX),
+        fees: u32::try_from(p.fees)
+            .map_err(|_| format!("pool fee {} exceeds u32::MAX", p.fees))?,
     })
 }