Add TSAN fiber-switching support, sanitizer/valgrind bazel configs, and CI

[dallison]

Summary

Re-opens the work from the (now-reverted) PR #32 with clean commit messages
(no Co-authored-by: Cursor <[email protected]> trailers, since BCR
rejects those). Same content, single squashable commit on top of
081378c.

• Teach the coroutine library to cooperate with ThreadSanitizer via its fiber
  API (__tsan_create_fiber / __tsan_switch_to_fiber / ...), in the same
  spirit as the existing AddressSanitizer integration. Every cooperative
  context switch now tells TSan which fiber it is leaving and which it is
  entering, so yields between the scheduler and its coroutines no longer look
  like data races.
• Add three reusable Bazel --config= profiles in .bazelrc for running the
  test suite under sanitizers / Memcheck:
  • --config=asan — -fsanitize=address with frame pointers and debug info.
  • --config=tsan — -fsanitize=thread with frame pointers and debug info.
  • --config=valgrind — -O1 -g, runs each test via --run_under with
    valgrind --error-exitcode=1 --leak-check=full --track-origins=yes ....
• Add a GitHub Actions CI workflow (.github/workflows/ci.yml) modeled on
  the one used by dallison/subspace,
  extended to exercise every config above on every push and pull request.

Implementation notes

TSan integration

• detect_sanitizers.h gains CO_THREAD_SANITIZER (auto-detected from
  __has_feature(thread_sanitizer) for clang and __SANITIZE_THREAD__ for
  gcc) and a matching CO_DISABLE_THREAD_SANITIZER attribute.
• coroutine.h forward-declares the five TSan fiber entry points we use and
  adds a tsan_fiber_ member to both Coroutine and CoroutineScheduler.
• coroutine.cc:
  • Adds a CO_TSAN_SWITCH_TO_FIBER(fiber) helper that compiles to
    __tsan_switch_to_fiber(fiber, 0) under TSan and a no-op otherwise.
  • Wires the helper into all three SWAPCONTEXT macro variants
    (setjmp / ucontext / custom-context) and into every SETCONTEXT(resume_),
    SETCONTEXT(exit_), and the tail of InvokeFunction.
  • Creates and names a per-coroutine fiber in the constructor and destroys
    it in the destructor.
  • Captures the scheduler thread's fiber lazily on the first call to Run(),
    since the scheduler may have been constructed on a different thread.
  • Annotates Coroutine::Resume with CO_DISABLE_THREAD_SANITIZER (it
    contains hand-written stack-switching assembly that TSan should not try
    to instrument) alongside the existing CO_DISABLE_ADDRESS_SANITIZER.

CI workflow

.github/workflows/ci.yml defines three jobs, each using
bazel-contrib/[email protected] with bazelisk / disk / repository caching:

Job	Runners	What it runs
Build & test (matrix)	ubuntu-latest, ubuntu-24.04-arm, macos-latest (with --config=apple_silicon)	bazel build //... then bazel test //...
sanitizers (matrix: asan, tsan)	ubuntu-latest	bazel test --config=$config //...
valgrind	ubuntu-latest	Installs valgrind via apt, then bazel test --config=valgrind //...

Failed runs upload bazel-testlogs/ as artifacts for post-mortem.

Test plan

• bazel build //co:co //co:cotest //co:costress //co:coroutines_test — passes locally.
• bazel test --config=asan //co:coroutines_test //co:test_cpp20 — both pass locally.
• bazel test --config=tsan //co:coroutines_test //co:test_cpp20 — both pass locally.
• bazel build --config=valgrind //co:coroutines_test //co:test_cpp20 — builds clean locally (running under Valgrind requires Linux).
• CI on the previous (reverted) iteration of this work passed all six jobs (Build & test (macos-latest / ubuntu-24.04-arm / ubuntu-latest), asan, tsan, valgrind); CI on this PR will reproduce the run.

Notes for review

• This branch contains a single commit; squash-merge or merge as you prefer.
  Either way the resulting commit message will be clean (no Cursor trailer).
• After merge, please cut a new release / version bump separately rather than
  relying on the dangling 3.3.1 / 3.3.2 tags from the previous attempt.