Wire RISC-V QEMU smoke test into pull.yml CI

Adds the reusable _test_riscv_qemu.yml workflow_call (mirroring
_test_cortex_m_e2e.yml) and a test-riscv-qemu job in pull.yml that calls
it on every PR. The job runs on a stock linux.2xlarge x86_64 runner
(no special hardware), apt-installs gcc-riscv64-linux-gnu and
qemu-user-static via examples/riscv/setup.sh, then invokes
.ci/scripts/test_riscv_qemu.sh which exports a small BundledProgram,
cross-compiles executor_runner for riscv64-linux, runs it under
qemu-user-static, and asserts the Test_result: PASS marker emitted by
the bundled-IO comparison path.

This is the CI half of the Phase 1.0 stepping stone toward
pytorch#18991; the toolchain, preset, AOT script, and run.sh
landed in the previous commit.

Co-authored-with: Claude (https://claude.ai/code)

Author: luhenry

.github/workflows/_test_riscv_qemu.yml

@@ -0,0 +1,32 @@
+name: Test RISC-V QEMU smoke
+
+permissions:
+  id-token: write
+  contents: read
+
+on:
+  workflow_call:
+    inputs:
+      timeout:
+        description: 'Per-job timeout in minutes'
+        required: false
+        type: number
+        default: 30
+
+jobs:
+  run:
+    uses: pytorch/test-infra/.github/workflows/linux_job_v2.yml@main
+    with:
+      runner: linux.2xlarge
+      docker-image: ci-image:executorch-ubuntu-22.04-clang12
+      submodules: 'recursive'
+      ref: ${{ github.event_name == 'pull_request' && github.event.pull_request.head.sha || github.sha }}
+      timeout: ${{ inputs.timeout }}
+      script: |
+        CONDA_ENV=$(conda env list --json | jq -r ".envs | .[-1]")
+        conda activate "${CONDA_ENV}"
+
+        source .ci/scripts/utils.sh
+        install_executorch "--use-pt-pinned-commit"
+
+        bash .ci/scripts/test_riscv_qemu.sh

.github/workflows/pull.yml

@@ -607,6 +607,13 @@ jobs:
           exit 1
         fi
 
+  test-riscv-qemu:
+    name: test-riscv-qemu
+    permissions:
+      id-token: write
+      contents: read
+    uses: ./.github/workflows/_test_riscv_qemu.yml
+
   test-mcu-cortex-m-backend:
     name: test-mcu-cortex-m-backend
     uses: pytorch/test-infra/.github/workflows/linux_job_v2.yml@main