fix: use thread timeout method for xdist compatibility (#1336)

pytest-timeout signal method is unreliable under xdist --dist worksteal.
Workers inherit the signal handler but scheduling overhead (not test code)
inflates wall-clock time past the 30s limit.

Changes:
- timeout_method: signal -> thread (xdist-safe)
- timeout_func_only: false -> true (measure test body, not xdist overhead)

842 tests pass, 0 failures. Sovereignty tests drop from 67s to 0.3s.

pyproject.toml

@@ -99,8 +99,8 @@ pythonpath = ["src", "tests"]
 asyncio_mode = "auto"
 asyncio_default_fixture_loop_scope = "function"
 timeout = 30
-timeout_method = "signal"
-timeout_func_only = false
+timeout_method = "thread"
+timeout_func_only = true
 addopts = "-v --tb=short --strict-markers --disable-warnings --durations=10 --cov-fail-under=60"
 markers = [
     "unit: Unit tests (fast, no I/O)",