libghostty: -Dvt-features to compile out unused features

This introduces a `-Dvt-features` build option for libghostty-vt that
compiles out optional feature areas, primarily so size-conscious
embedders (e.g. wasm) can significantly trim the binary.

The flag is similar to `-Dcpu`, `+feature` or `feature` to enable it,
`-feature` to disable, magic word `all` to turn all features on or off.
Example: `-Dvt-features=-all,+render-state` builds only the render
state API.

### Sizes

| Build | Bytes | Brotli |
|---|---|---|
| default (all features) | 876,500 | 218,309 |
| web interactive (`-all,+render-state,+input-encode,+selection,+color,+grid-introspection`) | 661,119 | 168,994 |
| read-only viewer (`-all,+render-state`) | 537,441 | 132,858 |
| bare VT core (`-all`) | 515,422 | 125,756 |
| xterm.js browser bundle (incl. renderers) | 488,663 | 99,311 |
| @xterm/headless | 182,672 | 39,651 |

Note: xterm versions are stable as of this commit.

.github/workflows/test.yml

@@ -95,6 +95,7 @@ jobs:
       - build-flatpak
       - build-libghostty-vt
       - build-libghostty-vt-wasm
+      - build-libghostty-vt-features
       - build-libghostty-vt-android
       - build-libghostty-vt-macos
       - build-libghostty-vt-windows
@@ -726,6 +727,52 @@ jobs:
             echo "Verified ${artifact} requires simd128"
           done
 
+  # Compile-only checks for the -Dvt-features flags so that future changes
+  # don't regress any feature combination.
+  build-libghostty-vt-features:
+    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@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+
+      - name: Setup Cache
+        uses: namespacelabs/nscloud-cache-action@c5f8dab7560444c4bf8dbc64f1b203431873c547 # v1.6.1
+        with:
+          path: |
+            /nix
+            /zig
+
+      # Install Nix and use that to run our tests so our environment matches exactly.
+      - uses: cachix/install-nix-action@630ae543ea3a38a9a4166f03376c02c50f408342 # v31.11.0
+        with:
+          nix_path: nixpkgs=channel:nixos-unstable
+      - uses: cachix/cachix-action@5f2d7c5294214f71b873db4b969586b980625e71 # v17
+        with:
+          name: ghostty
+          authToken: "${{ secrets.CACHIX_AUTH_TOKEN }}"
+
+      - name: Compile feature flag matrix
+        run: |
+          features="snapshot formatter selection render_state input_encode color grid_introspection glyph_protocol kitty_graphics"
+          configs="-all"
+          for f in $features; do configs="$configs -all,+$f"; done
+
+          for cfg in $configs; do
+            echo "::group::-Dvt-features=$cfg"
+            # The wasm lib build compiles the C API export block; the
+            # test build compiles every module test without running any.
+            nix develop -c zig build -Demit-lib-vt \
+              "-Dvt-features=$cfg" \
+              -Dtarget=wasm32-freestanding
+            nix develop -c zig build test-lib-vt-build \
+              "-Dvt-features=$cfg"
+            echo "::endgroup::"
+          done
+
   # lib-vt requires macOS runner for macOS/iOS builds because it requires the `apple_sdk` path
   build-libghostty-vt-macos:
     strategy: