examples: tennis market monitor with live match-state overlay

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What this adds

examples/tennis_market_monitor.py — a new runnable example that mirrors examples/simple_monitor.py. It watches Polymarket tennis events through PolyTerm's own GammaClient and overlays an independent, read-only live match-state annotation (score, who's serving, break point) next to each market.

It follows the existing example pattern exactly:

• standalone script under examples/, Config() + GammaClient(...) construction like simple_monitor.py;
• reuses polyterm.api.market_utils.market_probability_price / parse_list_field for the Polymarket side (no new market-data path);
• read-only: it places no orders and imports no execution code — the live feed is only a display overlay.

Data sources

• Polymarket market data: PolyTerm's GammaClient (unchanged; the sole market-data source here).
• Live match state: the Live Tennis API free tier (GET /matches?status=live), used purely as an external score/status annotation.

Vendor disclosure

I run the Live Tennis API (https://livetennisapi.com), so this example is vendor-authored — please judge accordingly. It positions the feed as an independent live-score/status source for debugging/annotating a market's inputs, never as an oracle, venue, or resolution source; the module docstring tells readers to confirm official resolution criteria on the Polymarket page. The free keyed tier (https://livetennisapi.com/subscribe/free) covers live matches + scores at 30 req/min, 100 req/day — enough for a develop-and-test or ~15-minute-cadence check, not continuous fast polling. The overlay is opt-in via LIVETENNIS_API_KEY; with no key set, the script prints the Polymarket side only. Happy to hand a maintainer a free key to try it.

Checks run locally

• Repo's CI syntax check over polyterm/ — passes.
• Example parses and all imports resolve in a fresh venv (pip install -e .).
• The derive_break_point helper is verified offline against the documented convention (receiver at AD, or receiver at 40 vs server 0/15/30; never in a tiebreak; null-safe on null server/points).
• pytest tests/ --ignore=tests/test_live_data --ignore=tests/test_tui/test_integration.py → 1116 passed, 1 skipped. (The 11 test_agent_mcp.py failures are a pre-existing mcp-package version guard in polyterm/agent/mcp/fastmcp_server.py, unrelated to this change — the new file is only added under examples/ and is imported by no test.)

grep -riE "casino|gambling" over the diff is empty.