libghostty: add ABI manifest schema

The ABI manifest previously had no machine-readable grammar or test that
the public export conformed to it.

Define a Draft 2020-12 schema and add a build check that executes
ghostty_type_json for native and wasm libraries before validation. Run
both forms in CI and publish the schema with the generated API docs.

.github/workflows/test.yml

@@ -701,6 +701,12 @@ jobs:
             -Dtarget=wasm32-freestanding \
             -Doptimize=ReleaseSmall
 
+      - name: Validate WASM ABI manifest
+        run: |
+          nix develop -c zig build test-lib-vt-schema \
+            -Dtarget=wasm32-freestanding \
+            -Doptimize=ReleaseSmall
+
       - name: Optimize ReleaseSmall WASM
         run: |
           nix develop -c wasm-opt -O3 \
@@ -1470,6 +1476,9 @@ jobs:
       - name: Test
         run: nix develop -c zig build test-lib-vt
 
+      - name: Validate native ABI manifest
+        run: nix develop -c zig build test-lib-vt-schema
+
   test-kaitai:
     if: github.repository == 'ghostty-org/ghostty' && needs.skip.outputs.skip != 'true'
     needs: skip