ci: check the client for aarch64 on every push

Adds a `rust-arm64` job: clippy -D warnings for the client crates at
--target aarch64-unknown-linux-gnu, plus a --no-default-features session
build so the minimal embedded configuration keeps compiling in its own
right rather than only as a subset of the default one. Runs in the cross
image on the ordinary amd64 runner; client crates are listed explicitly
because --workspace would drag in the x86-only host encode stack.

This is a regression guard, not an artifact leg (deb.yml ships those).
c_char signedness cuts both ways and neither direction is visible from an
x86-only CI: hardcoding i8 fails to COMPILE on ARM, while casting to u8
fails the LINT on ARM. Two such defects were already found this way.

Also documents the arm64 client .deb recipe in packaging/debian/README.md.

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

.gitea/workflows/ci.yml

@@ -108,6 +108,53 @@ jobs:
           git diff --exit-code include/punktfunk_core.h \
             || (echo "include/punktfunk_core.h is stale — commit the regenerated header" && exit 1)
 
+  # The client stack cross-checked for aarch64. NOT an artifact job — deb.yml ships those —
+  # this exists so a portability defect fails here instead of surfacing in a release build or
+  # on a user's board. It earns its runtime: the bug that motivated it (a Vulkan extension
+  # array typed `*const i8`, where `c_char` is signed on x86_64 and UNSIGNED on aarch64)
+  # compiled cleanly on every target CI built at the time.
+  #
+  # Client crates only, listed explicitly: the host's encode stack is x86 (NVENC/QSV/AMF) and
+  # `--workspace` would drag it in. Runs in the cross image (amd64 toolchain + arm64 sysroot,
+  # ci/rust-ci-arm64cross.Dockerfile) on the ordinary runner — no arm64 runner involved.
+  rust-arm64:
+    runs-on: ubuntu-24.04
+    container:
+      image: git.unom.io/unom/punktfunk-rust-ci-arm64cross:latest
+    timeout-minutes: 60
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Cache keys
+        run: echo "rustc=$(rustc --version | cut -d' ' -f2)" >> "$GITHUB_ENV"
+      - uses: actions/cache@v4
+        with:
+          path: |
+            /usr/local/cargo/registry
+            /usr/local/cargo/git
+          key: cargo-home-${{ hashFiles('Cargo.lock') }}
+          restore-keys: cargo-home-
+      - uses: actions/cache@v4
+        with:
+          path: target
+          # Its OWN prefix: aarch64 artifacts must never share the amd64 jobs' target cache.
+          key: cargo-target-arm64-v1-${{ env.rustc }}-${{ hashFiles('Cargo.lock') }}
+          restore-keys: cargo-target-arm64-v1-${{ env.rustc }}-
+
+      - name: Clippy for aarch64 (deny warnings)
+        run: |
+          cargo clippy --target aarch64-unknown-linux-gnu --all-targets --locked \
+            -p punktfunk-core -p pf-client-core -p pf-presenter -p pf-console-ui \
+            -p punktfunk-client-session -p punktfunk-client-linux \
+            -- -D warnings
+
+      # The minimal embedded build — no Skia, no PyroWave — is what a small image installs, so
+      # it has to keep compiling on its own, not just as a subset of the default features.
+      - name: Build the session binary, minimal features
+        run: |
+          cargo build --release --target aarch64-unknown-linux-gnu --locked \
+            -p punktfunk-client-session --no-default-features
+
   web:
     runs-on: ubuntu-24.04
     container: