Add macOS 26 (Tahoe) compat tests, skip zig build via stub (#1590)

Zig 0.15.2's MachO linker can't resolve libSystem on macOS 26 (the
version number jump from 15 to 26 breaks zig's SDK handling). The unit
tests don't need the CLI helper binary at runtime, so we skip the zig
build on macOS 26 by setting CMUX_SKIP_ZIG_BUILD=1, which creates a
stub binary to satisfy the Xcode Run Script file check.

Smoke test (full app build + launch) is skipped on macOS 26 since it
needs the real CLI helper.

Co-authored-by: Lawrence Chen <lawrencecchen@users.noreply.github.com>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.github/workflows/ci-macos-compat.yml

@@ -17,6 +17,11 @@ jobs:
           - os: warp-macos-15-arm64-6x
             timeout: 20
             smoke: true
+            skip_zig: false
+          - os: warp-macos-26-arm64-6x
+            timeout: 20
+            smoke: false
+            skip_zig: true  # zig 0.15.2 MachO linker can't resolve libSystem on macOS 26
     runs-on: ${{ matrix.os }}
     timeout-minutes: ${{ matrix.timeout }}
     steps:
@@ -62,6 +67,7 @@ jobs:
           ./scripts/download-prebuilt-ghosttykit.sh
 
       - name: Install zig
+        if: ${{ !matrix.skip_zig }}
         run: |
           ZIG_REQUIRED="0.15.2"
           if command -v zig >/dev/null 2>&1 && zig version 2>/dev/null | grep -q "^${ZIG_REQUIRED}"; then
@@ -114,6 +120,8 @@ jobs:
           done
 
       - name: Run unit tests
+        env:
+          CMUX_SKIP_ZIG_BUILD: ${{ matrix.skip_zig && '1' || '0' }}
         run: |
           set -euo pipefail
           SOURCE_PACKAGES_DIR="$PWD/.ci-source-packages"