Check the API level of an object a linker actually produced

The step built `-p dr-gpu` and then looked for a `*.so` to read the API level
out of with `file`. dr-gpu declares no `crate-type`, so it produces an rlib —
an archive of object files no linker has touched, and about which `file` has
nothing to say. The step could therefore only ever reach its own "no aarch64
.so was produced" branch, whatever the linker did, which is the one thing it
exists to rule out.

Builds `darkroom-android` instead: the workspace's only
`crate-type = ["cdylib"]`, and the artefact that ships in the APK. The API
level verified is now the one a device will refuse to install against.

The neighbouring comment claiming only core crates cross-compile is left
alone until the build proves otherwise; it is either stale or this commit is
wrong, and the same run answers both.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.gitea/workflows/build-and-test.yml

@@ -117,7 +117,18 @@ jobs:
           CARGO_TARGET_DIR: target-android
         run: |
           set -e
-          cargo ndk -t arm64-v8a build -p dr-gpu --release
+          # `darkroom-android`, not a core crate: this step reads the API level
+          # out of a *linked* object, and only that crate produces one. It is
+          # the workspace's single `crate-type = ["cdylib"]`; a library crate
+          # builds an rlib, which is an archive of object files that no linker
+          # has yet touched and that `file` therefore has nothing to say about.
+          # Asking for `-p dr-gpu` here could only ever reach the "no aarch64
+          # .so was produced" branch below, whatever the linker did.
+          #
+          # It is also the honest artefact to check: the .so this names is the
+          # one that ships in the APK, so the API level verified here is the
+          # API level a device will refuse to install against.
+          cargo ndk -t arm64-v8a build -p darkroom-android --release
           MIN_API=$(sed -n 's/^ARG MIN_API=\([0-9]*\).*/\1/p' docker/android/Dockerfile)
           # Empty on both sides would compare equal and pass, so neither side
           # is allowed to be the result of a failed parse.