Fix external brightness delays with multiple displays

.gitignore

@@ -11,6 +11,7 @@ DerivedData/
 Crisp.xcodeproj/
 Crisp.dmg
 .superpowers/
+.worktrees/
 Crisp-bin
 
 # Working/internal docs kept local-only (research, release-note drafts)

Crisp/Models/DDCOperationGeneration.swift

@@ -0,0 +1,37 @@
+import CoreGraphics
+
+struct DDCOperationGeneration {
+    struct Token: Equatable {
+        let topology: UInt64
+        let request: UInt64
+    }
+
+    private var topologies: [CGDirectDisplayID: UInt64] = [:]
+    private var requests: [CGDirectDisplayID: UInt64] = [:]
+
+    func currentToken(for displayID: CGDirectDisplayID) -> Token {
+        Token(
+            topology: topologies[displayID, default: 0],
+            request: requests[displayID, default: 0]
+        )
+    }
+
+    mutating func nextRequest(for displayID: CGDirectDisplayID) -> Token {
+        requests[displayID, default: 0] &+= 1
+        return currentToken(for: displayID)
+    }
+
+    mutating func invalidate(displayID: CGDirectDisplayID) {
+        topologies[displayID, default: 0] &+= 1
+        requests[displayID, default: 0] &+= 1
+    }
+
+    func isCurrentTopology(_ token: Token, for displayID: CGDirectDisplayID) -> Bool {
+        token.topology == topologies[displayID, default: 0]
+    }
+
+    func isLatestRequest(_ token: Token, for displayID: CGDirectDisplayID) -> Bool {
+        isCurrentTopology(token, for: displayID)
+            && token.request == requests[displayID, default: 0]
+    }
+}

Crisp/Models/DDCOperationQueuePool.swift

@@ -0,0 +1,22 @@
+import Foundation
+import CoreGraphics
+
+/// Keeps DDC operations serial per physical display without coupling displays.
+final class DDCOperationQueuePool: @unchecked Sendable {
+    private let lock = NSLock()
+    // ponytail: keep this small map for the process lifetime so a reused display ID
+    // cannot overlap an in-flight operation on a second queue.
+    private var queues: [CGDirectDisplayID: DispatchQueue] = [:]
+
+    func queue(for displayID: CGDirectDisplayID) -> DispatchQueue {
+        lock.withLock {
+            if let queue = queues[displayID] { return queue }
+            let queue = DispatchQueue(
+                label: "com.crisp.ddc.\(displayID)",
+                qos: .userInitiated
+            )
+            queues[displayID] = queue
+            return queue
+        }
+    }
+}

Crisp/Models/DDCServiceMatcher.swift

@@ -20,11 +20,20 @@ enum DDCServiceMatcher {
     /// A display's vendor/product/serial identity mirrors the IORegistry
     /// `ProductAttributes` (`LegacyManufacturerID` / `ProductID` / `SerialNumber`)
     /// which line up with `CGDisplayVendorNumber` / `CGDisplayModelNumber` /
-    /// `CGDisplaySerialNumber` for the same physical display.
+    /// `CGDisplaySerialNumber` for the same physical display. Location is the
+    /// stable CoreDisplay/IORegistry path when macOS exposes it.
     struct Identity: Equatable {
         let vendor: UInt32
         let product: UInt32
         let serial: UInt32
+        let location: String?
+
+        init(vendor: UInt32, product: UInt32, serial: UInt32, location: String? = nil) {
+            self.vendor = vendor
+            self.product = product
+            self.serial = serial
+            self.location = location
+        }
     }
 
     /// The outcome of a matching pass.
@@ -53,11 +62,17 @@ enum DDCServiceMatcher {
         var usedDisplays = Set<CGDirectDisplayID>()
         var unmatched: [Int] = []
 
-        // Strategy 1: identity matching (vendor+product+serial, then vendor+product).
+        // Strategy 1: stable location, then non-zero serial, then model identity.
         for i in services.indices {
             guard let idty = services[i] else { unmatched.append(i); continue }
-            let exact = displays.first {
+            let byLocation = displays.first {
+                guard let location = idty.location, !location.isEmpty else { return false }
+                return !usedDisplays.contains($0.id) && $0.identity.location == location
+            }
+            let exact = byLocation ?? displays.first {
                 !usedDisplays.contains($0.id)
+                    && idty.serial != 0
+                    && $0.identity.serial != 0
                     && $0.identity.vendor == idty.vendor
                     && $0.identity.product == idty.product
                     && $0.identity.serial == idty.serial