build: add pkg-config static linking support and fat archives to libghostty

The libghostty-vt pkg-config file was missing Libs.private, so
pkg-config --libs --static returned the same flags as the shared
case, omitting the C++ standard library needed by the SIMD code.

Additionally, the static archive did not bundle the vendored SIMD
dependencies (simdutf, highway, utfcpp), leaving consumers with
unresolved symbols when linking. If we're choosing to vendor (no -fsys)
then we should produce a fat static archive that includes them. If `-fsys`
is used, then we should not bundle them and instead reference them via
Requires.private, letting pkg-config chain to their own .pc files.

Add Libs.private with the C++ runtime (-lc++ on Darwin, -lstdc++
on Linux) and Requires.private for any SIMD deps provided via
system integration. When SIMD deps are vendored (the default),
produce a fat static archive that bundles them using libtool on
Darwin and ar on Linux. When they come from the system (-fsys=),
reference them via Requires.private instead, letting pkg-config
chain to their own .pc files.

.github/workflows/test.yml

@@ -108,6 +108,7 @@ jobs:
       - test-i18n
       - test-fuzz-libghostty
       - test-lib-vt
+      - test-lib-vt-pkgconfig
       - test-macos
       - test-windows
       - pinact
@@ -327,6 +328,116 @@ jobs:
           ls -la zig-out/lib/
           ls -la zig-out/include/ghostty/
 
+  test-lib-vt-pkgconfig:
+    runs-on: namespace-profile-ghostty-sm
+    needs: test
+    env:
+      ZIG_LOCAL_CACHE_DIR: /zig/local-cache
+      ZIG_GLOBAL_CACHE_DIR: /zig/global-cache
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Setup Cache
+        uses: namespacelabs/nscloud-cache-action@a90bb5d4b27522ce881c6e98eebd7d7e6d1653f9 # v1.4.2
+        with:
+          path: |
+            /nix
+            /zig
+
+      # Install Nix and use that to run our tests so our environment matches exactly.
+      - uses: cachix/install-nix-action@96951a368ba55167b55f1c916f7d416bac6505fe # v31.10.3
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - uses: cachix/cachix-action@1eb2ef646ac0255473d23a5907ad7b04ce94065c # v17
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
+      - name: Build libghostty-vt
+        run: nix develop -c zig build -Demit-lib-vt
+
+      - name: Verify pkg-config file
+        run: |
+          export PKG_CONFIG_PATH="$PWD/zig-out/share/pkgconfig"
+          pkg-config --validate libghostty-vt
+          echo "Cflags: $(pkg-config --cflags libghostty-vt)"
+          echo "Libs:   $(pkg-config --libs libghostty-vt)"
+          echo "Static: $(pkg-config --libs --static libghostty-vt)"
+
+          # Libs.private must include the C++ standard library
+          pkg-config --libs --static libghostty-vt | grep -q -- '-lc++'
+
+      - name: Verify static archive contains SIMD deps
+        run: |
+          nm -g zig-out/lib/libghostty-vt.a | grep -q ' T .*simdutf'
+          nm -g zig-out/lib/libghostty-vt.a | grep -q ' T .*3hwy'
+
+      - name: Write test program
+        run: |
+          cat > /tmp/test_libghostty_vt.c << 'TESTEOF'
+          #include <ghostty/vt.h>
+          #include <stdio.h>
+          int main(void) {
+              bool simd = false;
+              GhosttyResult r = ghostty_build_info(GHOSTTY_BUILD_INFO_SIMD, &simd);
+              if (r != GHOSTTY_SUCCESS) return 1;
+              printf("SIMD: %s\n", simd ? "yes" : "no");
+              return 0;
+          }
+          TESTEOF
+
+      - name: Test shared link via pkg-config
+        run: |
+          export PKG_CONFIG_PATH="$PWD/zig-out/share/pkgconfig"
+          nix develop -c cc -o /tmp/test_shared /tmp/test_libghostty_vt.c \
+            $(pkg-config --cflags --libs libghostty-vt) \
+            -Wl,-rpath,"$PWD/zig-out/lib"
+          /tmp/test_shared
+
+      - name: Test static link via pkg-config
+        run: |
+          export PKG_CONFIG_PATH="$PWD/zig-out/share/pkgconfig"
+          # The static library is compiled with LLVM libc++ (not GNU
+          # libstdc++), so linking requires a libc++-compatible toolchain.
+          # zig cc, clang, or gcc with libc++-dev installed all work.
+          nix develop -c zig cc -o /tmp/test_static /tmp/test_libghostty_vt.c \
+            $(pkg-config --cflags libghostty-vt) \
+            "$PWD/zig-out/lib/libghostty-vt.a" \
+            $(pkg-config --libs-only-l --static libghostty-vt | sed 's/-lghostty-vt//')
+          /tmp/test_static
+          # Verify it's truly statically linked (no libghostty-vt.so dependency)
+          ! ldd /tmp/test_static 2>/dev/null | grep -q libghostty-vt
+
+      # Test system integration: rebuild with -Dsystem-simdutf=true so
+      # simdutf comes from the system instead of being vendored. This
+      # verifies the .pc file uses Requires.private for system deps and
+      # the fat archive only bundles the remaining vendored deps.
+      - name: Rebuild with system simdutf
+        run: |
+          rm -rf zig-out
+          nix develop -c zig build -Demit-lib-vt -fsys=simdutf
+
+      - name: Verify pkg-config with system simdutf
+        run: |
+          export PKG_CONFIG_PATH="$PWD/zig-out/share/pkgconfig"
+          pc_content=$(cat zig-out/share/pkgconfig/libghostty-vt.pc)
+          echo "$pc_content"
+
+          # Requires.private must reference simdutf
+          echo "$pc_content" | grep -q 'Requires.private:.*simdutf'
+
+          # Requires.private must NOT reference libhwy (still vendored)
+          ! echo "$pc_content" | grep -q 'Requires.private:.*libhwy'
+
+      - name: Verify archive with system simdutf
+        run: |
+          # simdutf symbols must NOT be defined (comes from system)
+          ! nm -g zig-out/lib/libghostty-vt.a | grep -q ' T .*simdutf'
+
+          # highway symbols must still be defined (vendored)
+          nm -g zig-out/lib/libghostty-vt.a | grep -q ' T .*3hwy'
+
   build-flatpak:
     strategy:
       fail-fast: false