P0-16: Remove the five never-implemented OSC Fabric addresses from the schema and docs #194

Part of #175 (audit Phase 0). Decision D3: remove from schema, do not implement. Evidence: docs/audit/2026-11-09/detail/showcontrol-core.md §0.3, §1 OSCFabric; detail/docs.md §2 ADR-0007.

Why. ADR-0007 defines 11 addresses. Five have no sender and no receiver anywhere in the tree: /flowerbeds/mode, /captcha/mode, /pipes/mode, /captcha/restart, /captcha/game_started (the last is sent only by OSCFabric/test_client.py:107). Only TreeHouse runs an OSC server (osc_server.py); FlowerBeds main.py, FundingCAPTCHA app.py, and Pipes have no listener. Dashboard/serve.py:78 relays mode into nothing for three of four Elements. The five are documented as live in docs/FlowerBeds.md:103, docs/FundingCAPTCHA.md:158, docs/Dashboard.md:15,28, and ADR-0007:159-162.

Do.

• ADR-0007: strike the five rows; add a status note that mode control is TreeHouse-only by decision (2026-11-09).
• docs/FlowerBeds.md:103, docs/FundingCAPTCHA.md:158: remove the inbound rows. docs/Dashboard.md:15,28,74: state that the Dashboard controls TreeHouse only.
• ShowControl/Dashboard/serve.py: delete POST /api/mode and _load_network if nothing else needs it; drop python-osc from the Dashboard requirements if unused afterwards.
• OSCFabric/test_client.py: remove the send_event("game_started") test; keep send_event itself (used for /captcha/blowup).
• CONTEXT.md:111-120 table: already lists only inbound-to-TreeHouse rows; verify.

Done when. Every address in ADR-0007 has a grep-able sender and receiver; the Dashboard has no endpoint nobody calls.