feat(display): add display_control CoreGraphics binary, replace displayplacer as primary path

Derived from OSIT MasterKey app (LegacyUtilities.m, Ian Kohl 2019).
Uses CGConfigureDisplayMirrorOfDisplay — native macOS API, no Homebrew dependency.
Handler now tries display_control first; falls back to displayplacer for missing binary
or per-device configStr overrides. Build the binary via scripts/build-display-control.sh
on a Mac (requires Xcode CLT only), then commit resources/bin/display_control.

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

scripts/build-display-control.sh

@@ -0,0 +1,40 @@
+#!/bin/bash
+# scripts/build-display-control.sh
+# Compile the display_control binary for macOS.
+#
+# Requirements: Xcode Command Line Tools
+#   xcode-select --install
+#
+# Run this on a Mac. Commit the resulting binary to resources/bin/
+# so it is bundled into the packaged Electron app without any Homebrew dependency.
+
+set -e
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+REPO_ROOT="$(cd "$SCRIPT_DIR/.." && pwd)"
+SRC="$SCRIPT_DIR/display_control.m"
+OUT_DIR="$REPO_ROOT/resources/bin"
+OUT_BIN="$OUT_DIR/display_control"
+
+if ! command -v clang &>/dev/null; then
+    echo "ERROR: clang not found."
+    echo "Install Xcode Command Line Tools:  xcode-select --install"
+    exit 1
+fi
+
+mkdir -p "$OUT_DIR"
+
+echo "Building display_control..."
+clang -framework Cocoa -framework Carbon \
+    -o "$OUT_BIN" "$SRC"
+
+chmod +x "$OUT_BIN"
+
+echo "Built: $OUT_BIN"
+echo ""
+echo "Test it:"
+echo "  $OUT_BIN status"
+echo "  $OUT_BIN extend"
+echo "  $OUT_BIN mirror"
+echo ""
+echo "Once verified, commit resources/bin/display_control to the repo."

scripts/display_control.m

@@ -0,0 +1,149 @@
+/*
+ * display_control.m
+ * Native macOS CLI for programmatic display mirror/extend control.
+ *
+ * Derived from the OSIT MasterKey app (LegacyUtilities.m, Ian Kohl 2019).
+ * Uses CoreGraphics APIs — no external dependencies (no displayplacer/Homebrew required).
+ *
+ * Build: run scripts/build-display-control.sh on a Mac (requires Xcode Command Line Tools)
+ * Usage: display_control <mirror|extend|status>
+ */
+
+#import <Cocoa/Cocoa.h>
+#include <Carbon/Carbon.h>
+
+#define MAX_DISPLAYS 8
+
+static int mirror_displays(void) {
+    CGDirectDisplayID onlineDspys[MAX_DISPLAYS] = {0};
+    CGDisplayCount numOnline = 0;
+
+    CGGetOnlineDisplayList(MAX_DISPLAYS, onlineDspys, &numOnline);
+
+    if (numOnline < 2) {
+        fprintf(stderr, "No secondary display detected (%u online).\n", numOnline);
+        return 1;
+    }
+
+    CGDirectDisplayID mainID = CGMainDisplayID();
+
+    CGDisplayConfigRef config;
+    CGError err = CGBeginDisplayConfiguration(&config);
+    if (err != kCGErrorSuccess) {
+        fprintf(stderr, "CGBeginDisplayConfiguration failed: %d\n", err);
+        return 1;
+    }
+
+    BOOL any_configured = NO;
+    for (CGDisplayCount i = 0; i < numOnline; i++) {
+        if (onlineDspys[i] != mainID) {
+            err = CGConfigureDisplayMirrorOfDisplay(config, onlineDspys[i], mainID);
+            if (err != kCGErrorSuccess) {
+                fprintf(stderr, "CGConfigureDisplayMirrorOfDisplay failed: %d\n", err);
+                CGCancelDisplayConfiguration(config);
+                return 1;
+            }
+            any_configured = YES;
+        }
+    }
+
+    if (!any_configured) {
+        CGCancelDisplayConfiguration(config);
+        fprintf(stderr, "No secondary displays to mirror.\n");
+        return 1;
+    }
+
+    err = CGCompleteDisplayConfiguration(config, kCGConfigurePermanently);
+    if (err != kCGErrorSuccess) {
+        fprintf(stderr, "CGCompleteDisplayConfiguration failed: %d\n", err);
+        return 1;
+    }
+
+    printf("Mirrored %u display(s).\n", numOnline - 1);
+    return 0;
+}
+
+static int extend_displays(void) {
+    CGDirectDisplayID onlineDspys[MAX_DISPLAYS] = {0};
+    CGDirectDisplayID activeDspys[MAX_DISPLAYS] = {0};
+    CGDisplayCount numOnline = 0, numActive = 0;
+
+    CGGetOnlineDisplayList(MAX_DISPLAYS, onlineDspys, &numOnline);
+    CGGetActiveDisplayList(MAX_DISPLAYS, activeDspys, &numActive);
+
+    if (numOnline < 2) {
+        fprintf(stderr, "No secondary display detected (%u online).\n", numOnline);
+        return 1;
+    }
+
+    if (numActive >= numOnline) {
+        printf("Displays already extended.\n");
+        return 0;
+    }
+
+    CGDirectDisplayID mainID = CGMainDisplayID();
+
+    CGDisplayConfigRef config;
+    CGError err = CGBeginDisplayConfiguration(&config);
+    if (err != kCGErrorSuccess) {
+        fprintf(stderr, "CGBeginDisplayConfiguration failed: %d\n", err);
+        return 1;
+    }
+
+    for (CGDisplayCount i = 0; i < numOnline; i++) {
+        if (onlineDspys[i] != mainID) {
+            // kCGNullDirectDisplay as master = un-mirror (extend)
+            err = CGConfigureDisplayMirrorOfDisplay(config, onlineDspys[i], kCGNullDirectDisplay);
+            if (err != kCGErrorSuccess) {
+                fprintf(stderr, "CGConfigureDisplayMirrorOfDisplay(null) failed: %d\n", err);
+                CGCancelDisplayConfiguration(config);
+                return 1;
+            }
+        }
+    }
+
+    err = CGCompleteDisplayConfiguration(config, kCGConfigurePermanently);
+    if (err != kCGErrorSuccess) {
+        fprintf(stderr, "CGCompleteDisplayConfiguration failed: %d\n", err);
+        return 1;
+    }
+
+    printf("Displays extended.\n");
+    return 0;
+}
+
+static void print_status(void) {
+    CGDirectDisplayID onlineDspys[MAX_DISPLAYS] = {0};
+    CGDirectDisplayID activeDspys[MAX_DISPLAYS] = {0};
+    CGDisplayCount numOnline = 0, numActive = 0;
+
+    CGGetOnlineDisplayList(MAX_DISPLAYS, onlineDspys, &numOnline);
+    CGGetActiveDisplayList(MAX_DISPLAYS, activeDspys, &numActive);
+
+    printf("online=%u active=%u %s\n",
+           numOnline, numActive,
+           (numOnline > 1 && numActive < numOnline) ? "mirrored" : "extended");
+}
+
+int main(int argc, const char * argv[]) {
+    @autoreleasepool {
+        if (argc < 2) {
+            fprintf(stderr, "Usage: display_control <mirror|extend|status>\n");
+            return 1;
+        }
+
+        const char *cmd = argv[1];
+
+        if (strcmp(cmd, "mirror") == 0) {
+            return mirror_displays();
+        } else if (strcmp(cmd, "extend") == 0) {
+            return extend_displays();
+        } else if (strcmp(cmd, "status") == 0) {
+            print_status();
+            return 0;
+        } else {
+            fprintf(stderr, "Unknown command: %s\nUsage: display_control <mirror|extend|status>\n", cmd);
+            return 1;
+        }
+    }
+}