ci: set RUST_TEST_THREADS=1 on musl to prevent concurrent PTY SIGSEGV

The PTY gate serializes Terminal lifetimes within pty_terminal, but the
SIGSEGV may occur in other concurrent operations (ctor init, signal
handlers). Setting test threads to 1 eliminates all concurrency.

Author: claude

.github/workflows/ci.yml

@@ -149,6 +149,9 @@ jobs:
       # -crt-static: vite-task is shipped as a NAPI module in vite+, and musl Node
       # with native modules links to musl libc dynamically, so we must do the same.
       RUSTFLAGS: --cfg tokio_unstable -D warnings -C target-feature=-crt-static
+      # On musl, concurrent PTY operations can trigger SIGSEGV in musl internals.
+      # Run test threads sequentially to avoid the race.
+      RUST_TEST_THREADS: 1
     steps:
       - name: Install Alpine dependencies
         shell: sh {0}

.github/workflows/musl-stability.yml

@@ -24,6 +24,7 @@ jobs:
       options: --shm-size=256m
     env:
       RUSTFLAGS: --cfg tokio_unstable -D warnings -C target-feature=-crt-static
+      RUST_TEST_THREADS: 1
     steps:
       - name: Install Alpine dependencies
         shell: sh {0}