v0.8.1 — Fix: MCP plot tools on macOS (Agg backend)

Fixed

• MCP plot tools no longer crash on macOS. tmelandscape-mcp now
  forces the Agg backend before any matplotlib import in
  tmelandscape.mcp.server, so the 11 figure-producing tools (plot_*)
  succeed when FastMCP dispatches them on worker threads. Previously, the
  default MacOSX GUI backend required the main thread and raised
  "Cannot create a GUI FigureManager outside the main thread..." on the
  first plt.figure() call. Reproduced 2026-06-04 against v0.8.0 on
  macOS 14.x while running the M00_S01_canary pipeline.

Unchanged (by design)

• tmelandscape.viz.* modules contain no matplotlib.use(...) calls.
  Direct Python callers (Jupyter, scripts) keep whatever backend they
  chose. The fix is isolated to the MCP server entry.
• Operator override is honoured: launching with
  MPLBACKEND=pdf tmelandscape-mcp writes PDFs instead of PNGs, since
  the server uses os.environ.setdefault(...) not a hard assignment.

Verification snapshot

• uv run pytest tests/unit tests/integration -q — 500 passed (was 499;
  the new test_mcp_server_forces_agg_backend adds one).
• uv run ruff check . / uv run ruff format --check . /
  uv run mypy src — clean.
• python -c "from tmelandscape.mcp.server import mcp; import matplotlib; print(matplotlib.get_backend())" — prints agg.

Known follow-ups (not addressed here)

• plot_state_feature_clustermap still fails on Leiden cluster-mean
  matrices that contain NaN entries (common when *_by_type spatial
  statistics are computed on cell types that start at population 0 and
  arise via transformations — e.g. M00's CD8 / M1 / M2 / exhausted-T).
  Open follow-up: decide whether the figure function skips NaN rows /
  columns automatically, emits a more actionable error, or whether
  summarize_ensemble drops all-NaN stats upstream. Flagged by the
  M00_S01_canary report; tracked separately.