build(macos): build the original Swift app against the Rust core

Ghostty's macOS app stays the original, unchanged Swift app (agreed
scope); it is wired to the Rust core instead of the Zig core.
scripts/macos-swift-app.sh (`just macos-swift-app <ghostty-src>`)
mirrors the Zig build's GhosttyXcodebuild.zig:
1. scripts/xcframework.sh builds the Rust libghostty (identical
   include/ghostty.h ABI) as <src>/macos/GhosttyKit.xcframework,
2. `just install <src>/zig-out` provides the resources the Xcode project
   bundles from ../zig-out/share (ghostty, terminfo, completions, man,
   locale, vim, nvim, bat),
3. xcodebuild -target Ghostty in a clean environment.
macOS + Xcode only; DRY_RUN=1 previews the steps (verified on Linux).

Test Plan:
- DRY_RUN=1 scripts/macos-swift-app.sh ../ghostty
- on macOS: just macos-swift-app ../ghostty && open the built app

scripts/macos-swift-app.sh

@@ -0,0 +1,55 @@
+#!/usr/bin/env bash
+# Build Ghostty's ORIGINAL Swift macOS app (unchanged) against the Rust core.
+#
+# Ghostty's Xcode project (<ghostty>/macos/Ghostty.xcodeproj) links
+# `macos/GhosttyKit.xcframework` and bundles resources from `../zig-out/share`
+# (see the Zig build's src/build/GhosttyXcodebuild.zig). The Rust libghostty
+# exports the identical include/ghostty.h ABI, so this script produces both
+# from the Rust workspace and then runs xcodebuild, exactly like
+# `zig build -Demit-macos-app`:
+#   1. scripts/xcframework.sh  -> <ghostty>/macos/GhosttyKit.xcframework
+#   2. `just install`          -> <ghostty>/zig-out/share/{ghostty,terminfo,...}
+#   3. xcodebuild -target Ghostty -configuration <CONFIG> (clean env, like Zig)
+#
+# Usage: scripts/macos-swift-app.sh <path-to-ghostty-source> [Release|Debug|ReleaseLocal]
+# Runs only on macOS (Xcode required). DRY_RUN=1 prints the commands instead.
+set -euo pipefail
+
+cd "$(dirname "$0")/.."
+src=${1:?usage: $0 <path-to-ghostty-source> [Release|Debug|ReleaseLocal]}
+config=${2:-Release}
+src=$(cd "$src" && pwd)
+[ -d "$src/macos/Ghostty.xcodeproj" ] || {
+    echo "error: $src/macos/Ghostty.xcodeproj not found" >&2
+    exit 1
+}
+
+run() {
+    if [ "${DRY_RUN:-}" = 1 ]; then
+        printf '+'
+        printf ' %q' "$@"
+        printf '\n'
+    else
+        "$@"
+    fi
+}
+
+if [ "${DRY_RUN:-}" != 1 ] && [ "$(uname -s)" != Darwin ]; then
+    echo "error: building the macOS app requires macOS with Xcode (use DRY_RUN=1 to preview)" >&2
+    exit 1
+fi
+
+# Debug configuration pairs with the dev profile (debug app id), like Zig.
+profile=production
+[ "$config" = Debug ] && profile=dev
+
+# 1. The Rust core as GhosttyKit.xcframework where the Xcode project expects it.
+run scripts/xcframework.sh universal "$profile" "$src/macos/GhosttyKit.xcframework"
+
+# 2. Resources the Xcode project copies from ../zig-out/share.
+run just install "$src/zig-out"
+
+# 3. xcodebuild with a clean environment (only PATH), as GhosttyXcodebuild.zig.
+run env -i PATH="$PATH" sh -c "cd '$src/macos' && xcodebuild -target Ghostty -configuration '$config'"
+
+echo "Ghostty.app: $src/macos/build/$config/Ghostty.app"