build: add CMake support for libghostty-vt (#11700)

Add a top-level CMakeLists.txt that wraps `zig build lib-vt` so that
CMake-based downstream projects can consume libghostty-vt without
needing to interact with the Zig build system directly. A custom command
triggers the zig build during `cmake --build`, and the resulting shared
library is exposed as an IMPORTED target.

Downstream projects can pull in the library via FetchContent, which
fetches the source and builds it as part of their own CMake build, or
via find_package after a manual install step. The package config
template in dist/cmake/ sets up the ghostty-vt::ghostty-vt target with
proper include paths and macOS rpath handling.

A c-vt-cmake example demonstrates the FetchContent workflow, creating a
terminal, writing VT sequences, and formatting the output as plain text.
CI is updated to auto-discover and build CMake-based examples alongside
the existing Zig-based ones.

> [!WARNING]
>
> I am **very much not a CMake expert.** I leaned on LLMs heavily for
this. I did read the docs for what was chosen here and understand what's
going on, but if there is a better or more idiomatic way to do this I'm
all ears!

## Example CMake File

```cmake
cmake_minimum_required(VERSION 3.19)
project(c-vt-cmake LANGUAGES C)

include(FetchContent)
FetchContent_Declare(ghostty
    GIT_REPOSITORY https://github.com/ghostty-org/ghostty.git
    GIT_TAG main
)
FetchContent_MakeAvailable(ghostty)

add_executable(c_vt_cmake src/main.c)
target_link_libraries(c_vt_cmake PRIVATE ghostty-vt)
```

.github/workflows/test.yml

@@ -85,7 +85,8 @@ jobs:
       - skip
       - build-bench
       - build-dist
-      - build-examples
+      - build-examples-zig
+      - build-examples-cmake
       - build-flatpak
       - build-libghostty-vt
       - build-libghostty-vt-android
@@ -172,22 +173,26 @@ jobs:
     needs: skip
     runs-on: namespace-profile-ghostty-xsm
     outputs:
-      dirs: ${{ steps.list.outputs.dirs }}
+      zig: ${{ steps.list.outputs.zig }}
+      cmake: ${{ steps.list.outputs.cmake }}
     steps:
       - name: Checkout code
         uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
       - id: list
         name: List example directories
         run: |
-          dirs=$(ls example/*/build.zig.zon 2>/dev/null | xargs -n1 dirname | xargs -n1 basename | jq -R -s -c 'split("\n") | map(select(. != ""))')
-          echo "$dirs" | jq .
-          echo "dirs=$dirs" >> "$GITHUB_OUTPUT"
+          zig=$(ls example/*/build.zig.zon 2>/dev/null | xargs -n1 dirname | xargs -n1 basename | jq -R -s -c 'split("\n") | map(select(. != ""))')
+          echo "$zig" | jq .
+          echo "zig=$zig" >> "$GITHUB_OUTPUT"
+          cmake=$(ls example/*/CMakeLists.txt 2>/dev/null | xargs -n1 dirname | xargs -n1 basename | jq -R -s -c 'split("\n") | map(select(. != ""))')
+          echo "$cmake" | jq .
+          echo "cmake=$cmake" >> "$GITHUB_OUTPUT"
 
-  build-examples:
+  build-examples-zig:
     strategy:
       fail-fast: false
       matrix:
-        dir: ${{ fromJSON(needs.list-examples.outputs.dirs) }}
+        dir: ${{ fromJSON(needs.list-examples.outputs.zig) }}
     name: Example ${{ matrix.dir }}
     runs-on: namespace-profile-ghostty-xsm
     needs: [test, list-examples]
@@ -219,6 +224,43 @@ jobs:
           cd example/${{ matrix.dir }}
           nix develop -c zig build
 
+  build-examples-cmake:
+    strategy:
+      fail-fast: false
+      matrix:
+        dir: ${{ fromJSON(needs.list-examples.outputs.cmake) }}
+    name: Example ${{ matrix.dir }}
+    runs-on: namespace-profile-ghostty-xsm
+    needs: [test, list-examples]
+    env:
+      ZIG_LOCAL_CACHE_DIR: /zig/local-cache
+      ZIG_GLOBAL_CACHE_DIR: /zig/global-cache
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
+
+      - name: Setup Cache
+        uses: namespacelabs/nscloud-cache-action@a90bb5d4b27522ce881c6e98eebd7d7e6d1653f9 # v1.4.2
+        with:
+          path: |
+            /nix
+            /zig
+
+      # Install Nix and use that to run our tests so our environment matches exactly.
+      - uses: cachix/install-nix-action@1ca7d21a94afc7c957383a2d217460d980de4934 # v31.10.1
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - uses: cachix/cachix-action@1eb2ef646ac0255473d23a5907ad7b04ce94065c # v17
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
+      - name: Build Example
+        run: |
+          cd example/${{ matrix.dir }}
+          nix develop -c cmake -B build -DFETCHCONTENT_SOURCE_DIR_GHOSTTY=${{ github.workspace }}
+          nix develop -c cmake --build build
+
   build-flatpak:
     strategy:
       fail-fast: false