ci: replace global RUST_TEST_THREADS=1 with per-crate --test-threads=1

On musl, fork() is unsafe when other threads exist — even threads
sleeping on a futex leave musl internal state that the child inherits
in an inconsistent form, causing SIGSEGV between fork and exec.

A process-wide mutex (PTY_LOCK) cannot fix this because the test
harness still spawns multiple threads (one per test). The real fix
is --test-threads=1 for PTY test binaries, which ensures only one
thread exists during fork.

Split `cargo test` into:
- Non-PTY tests: run in parallel (default thread count)
- PTY tests (pty_terminal, pty_terminal_test, vite_task_bin):
  run with --test-threads=1

This removes the global RUST_TEST_THREADS=1 that forced ALL tests
to run sequentially, improving CI speed for non-PTY tests.

https://claude.ai/code/session_011H8UR3gS6hoyQAf2x7Dfw8

Author: claude

.github/workflows/ci.yml

@@ -149,9 +149,6 @@ 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}
@@ -175,7 +172,15 @@ jobs:
           corepack enable
           pnpm install
 
-      - run: cargo test
+      # Run non-PTY tests in parallel (default thread count).
+      - run: cargo test --workspace --exclude pty_terminal --exclude pty_terminal_test --exclude vite_task_bin
+
+      # PTY tests must run single-threaded: musl's fork() is not safe when
+      # other threads exist (even sleeping on a futex), because musl internal
+      # state inherited by the child can be inconsistent.
+      - run: cargo test -p pty_terminal -- --test-threads=1
+      - run: cargo test -p pty_terminal_test -- --test-threads=1
+      - run: cargo test -p vite_task_bin -- --test-threads=1
 
   fmt:
     name: Format and Check Deps