ci: reuse the headless cmux-tui build in SDK conformance

.github/workflows/cmux-tui-sdks.yml

@@ -600,12 +600,38 @@ jobs:
         working-directory: cmux-tui/bindings/typescript
         run: npm ci --no-audit --no-fund
 
+      # The build reads only the cmux-tui tree, the ghostty submodule, and the
+      # zig and Rust toolchains pinned here and in cmux-tui/rust-toolchain.toml.
+      # Keyed on exactly those, an unchanged tree skips a five-minute compile,
+      # which is every run started by a workflow-only edit.
+      - name: Key the headless cmux-tui build
+        id: headless-key
+        run: |
+          set -euo pipefail
+          echo "key=cmux-tui-headless-v1-${RUNNER_OS}-${RUNNER_ARCH}-zig0.16.0-$(git rev-parse HEAD:cmux-tui)-$(git rev-parse HEAD:ghostty)" >> "$GITHUB_OUTPUT"
+
+      - name: Restore the headless cmux-tui build
+        id: headless-cache
+        uses: actions/cache/restore@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.5
+        with:
+          path: cmux-tui/target/debug/cmux-tui
+          key: ${{ steps.headless-key.outputs.key }}
+
       - name: Build exact headless cmux-tui
+        if: ${{ steps.headless-cache.outputs.cache-hit != 'true' }}
         working-directory: cmux-tui
         run: |
           test "$(zig version)" = "0.16.0"
           cargo build -p cmux-tui --bin cmux-tui --locked
 
+      # Pull request caches stay read-only; main publishes the build they reuse.
+      - name: Save the headless cmux-tui build
+        if: ${{ github.event_name == 'push' && steps.headless-cache.outputs.cache-hit != 'true' }}
+        uses: actions/cache/save@27d5ce7f107fe9357f9df03efb73ab90386fccae # v5.0.5
+        with:
+          path: cmux-tui/target/debug/cmux-tui
+          key: ${{ steps.headless-key.outputs.key }}
+
       - name: Set up Zig for SDK conformance
         uses: mlugg/setup-zig@8d6198c65fb0feaa111df26e6b467fea8345e46f # v2.0.5
         with: