ci: add temporary musl stability workflow (10 parallel runs)

Runs the full musl test suite 10 times in parallel to verify
the PTY serialization fix is stable.

https://claude.ai/code/session_011H8UR3gS6hoyQAf2x7Dfw8

Author: claude

.github/workflows/musl-stability.yml

@@ -0,0 +1,56 @@
+name: Musl Stability Test
+
+permissions:
+  contents: read
+
+on:
+  workflow_dispatch:
+  push:
+    branches:
+      - claude/reproduce-flaky-failure-RuwlG
+
+concurrency:
+  group: musl-stability-${{ github.sha }}
+  cancel-in-progress: true
+
+defaults:
+  run:
+    shell: bash
+
+jobs:
+  test-musl:
+    name: 'Musl Run #${{ matrix.run }}'
+    runs-on: ubuntu-latest
+    container:
+      image: node:22-alpine3.21
+      options: --shm-size=256m
+    env:
+      RUSTFLAGS: --cfg tokio_unstable -D warnings
+    strategy:
+      fail-fast: false
+      matrix:
+        run: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
+    steps:
+      - name: Install Alpine dependencies
+        shell: sh {0}
+        run: apk add --no-cache bash curl git musl-dev gcc g++ python3
+
+      - uses: actions/checkout@11bd71901bbe5b1630ceea73d27597364c9af683 # v4.2.2
+        with:
+          persist-credentials: false
+          submodules: true
+
+      - name: Install rustup
+        run: |
+          curl --proto '=https' --tlsv1.2 -sSf https://sh.rustup.rs | sh -s -- -y --default-toolchain none
+          echo "$HOME/.cargo/bin" >> "$GITHUB_PATH"
+
+      - name: Install Rust toolchain
+        run: rustup show
+
+      - name: Install pnpm and Node tools
+        run: |
+          corepack enable
+          pnpm install
+
+      - run: cargo test