fix: eliminate pandoc subprocess thrash under pytest-xdist -n auto

Root cause of recurring 35929 / 38536 CI flake — `-n auto` on a 32-core
CI runner spawns 32 pytest workers. test_pandoc.py contains ~14 tests
that each fork a `pandoc` subprocess. When several land on parallel
workers at once, pandoc's cold-start cost (GHC runtime + filter loading)
plus runner CPU contention pushes wall time past the 5s metadata timeout
and 30s convert timeout. Subprocess gets SIGKILL'd (returncode -9),
tests fail, pipeline reruns the same flake.

Three-part root cause fix:

1. Cache get_available_input_formats / get_available_output_formats with
   functools.lru_cache(maxsize=1). Pandoc's format list is static for
   a given binary; we only need one subprocess per Python process,
   not one per call site. Also bump the metadata-query timeout from 5s
   to 30s — the first cold-start under contention still has to succeed.

2. Mark TestConvert + TestAvailableFormats with
   @pytest.mark.xdist_group("pandoc-subprocess") so every pandoc-forking
   test pins to one xdist worker. Other 31 workers continue parallel-
   processing the rest of the suite; pandoc tests run sequentially on
   their single worker, each pandoc cold-start completes before the
   next starts.

3. Update Makefile test target to pass --dist=loadgroup so pytest-xdist
   honors the xdist_group marker.

Other test classes in test_pandoc.py (TestNodeTreeToMarkdown,
TestNamespaceToMarkdown, TestGetAuthorName) use MockNode objects
without data_html, so node_tree_to_markdown's pandoc path stays cold —
those classes don't need the marker.

Replaces 1527ef7 (which just retriggered the same flake).

Makefile

@@ -149,10 +149,14 @@ activate:
 	@echo "To activate the virtual environment, run:"
 	@echo "  source $(VENV_DIR)/bin/activate"
 
-# Run the test suite (installs test dependencies if needed)
+# Run the test suite (installs test dependencies if needed).
+# --dist=loadgroup pins tests sharing an xdist_group marker to a single
+# worker — used by test_pandoc.py to serialize ~14 pandoc subprocesses
+# that would otherwise race cold-start CPU contention on CI and exceed
+# the 5s/30s subprocess timeouts.
 test: install-source-dev-and-test
 	@echo "Running tests in parallel..."
-	$(VENV_DIR)/bin/py.test -n auto
+	$(VENV_DIR)/bin/py.test -n auto --dist=loadgroup
 
 # Start a simple HTTP server (for serving static files like index.html)
 http: venv

remarkbox/lib/pandoc.py

@@ -8,6 +8,7 @@ import logging
 import subprocess
 import tempfile
 import os
+from functools import lru_cache
 
 log = logging.getLogger(__name__)
 
@@ -155,12 +156,23 @@ def resolve_format(ext):
     return EXTENSION_ALIASES.get(ext, ext)
 
 
+# 30s — generous on cold-start contended runners (xdist N-worker fan-out).
+# The result is cached, so this cost gets paid at most once per process.
+_FORMAT_QUERY_TIMEOUT = 30
+
+
+@lru_cache(maxsize=1)
 def get_available_output_formats():
-    """Return the set of output formats pandoc supports on this system."""
+    """Return the set of output formats pandoc supports on this system.
+
+    Result cached per process — pandoc's format list is static for a given
+    binary, so subsequent callers (and parallel pytest workers each in their
+    own Python process) skip the subprocess entirely after first call.
+    """
     try:
         result = subprocess.run(
             ["pandoc", "--list-output-formats"],
-            capture_output=True, text=True, timeout=5,
+            capture_output=True, text=True, timeout=_FORMAT_QUERY_TIMEOUT,
         )
         return frozenset(result.stdout.strip().split("\n"))
     except Exception:
@@ -168,12 +180,16 @@ def get_available_output_formats():
         return frozenset()
 
 
+@lru_cache(maxsize=1)
 def get_available_input_formats():
-    """Return the set of input formats pandoc supports on this system."""
+    """Return the set of input formats pandoc supports on this system.
+
+    Result cached per process — see get_available_output_formats() above.
+    """
     try:
         result = subprocess.run(
             ["pandoc", "--list-input-formats"],
-            capture_output=True, text=True, timeout=5,
+            capture_output=True, text=True, timeout=_FORMAT_QUERY_TIMEOUT,
         )
         return frozenset(result.stdout.strip().split("\n"))
     except Exception:

remarkbox/tests/test_pandoc.py

@@ -1,4 +1,14 @@
-"""Unit tests for remarkbox.lib.pandoc — pandoc subprocess wrapper."""
+"""Unit tests for remarkbox.lib.pandoc — pandoc subprocess wrapper.
+
+xdist_group("pandoc-subprocess") on every class that fires pandoc as a
+subprocess. With -n auto on a 32-core CI runner, ~14 of these tests would
+otherwise race for `pandoc` cold-starts simultaneously, exceeding 5s/30s
+subprocess timeouts under GHC-runtime + filter-loading contention.
+Grouping pins every pandoc-subprocess test onto one worker, so each pandoc
+invocation completes before the next starts. Other workers keep parallel-
+processing the rest of the suite. Requires Makefile to pass
+--dist=loadgroup so pytest-xdist honors the marker.
+"""
 
 import shutil
 import unittest
@@ -19,6 +29,7 @@ from remarkbox.lib.pandoc import (
 )
 
 
+@pytest.mark.xdist_group("pandoc-subprocess")
 class TestConvert(unittest.TestCase):
     """Unit tests for the pandoc convert() function."""
 
@@ -95,6 +106,7 @@ class TestConvert(unittest.TestCase):
             convert("test", "markdown", "not_a_real_format_xyz")
 
 
+@pytest.mark.xdist_group("pandoc-subprocess")
 class TestAvailableFormats(unittest.TestCase):
     """Test format discovery."""