Mount Experimental Features dialog; add shared-server toggle; clean up flags

[TCVinNYC]

Summary

• Engine → Experimental Features… is back (the dialog existed, fully built and registry-driven, but nothing mounted it since the rc9 simplification pass — verified unreachable). Also linked from the Services panel, following the Manage Models… pattern.
• Shared detection server (AUTOPTZ_MODEL_SERVER) added to the curated registry — the validated many-camera scale mode is now a visible, persisted, restart-badged toggle instead of an env-only debug flag. Honest description including the new crash recovery. Still off by default.
• Dead flag removed: AUTOPTZ_INFERENCE_SCHEDULER was read nowhere in the app (only echoed into bench artifacts) — references and JSON fields deleted.
• Flag surface documented: new docs/flags.md table — every AUTOPTZ_* var categorized (Experimental — use the dialog / Operations override / Internal-dev), with defaults.
• Policy note appended to docs/engineering/retired-experiments.md owning the reversal (user decision for 2.2.0; the curated-not-generic shape still governs).

Test plan

• New offscreen tests: menu action exists + opens the dialog; dialog renders the model-server row; persisted-flag apply path round-trips AUTOPTZ_MODEL_SERVER; unmanaged-env baseline test rekeyed to a truly unmanaged var.
• Full suite + ruff + mypy + selftest green locally (run post-soak on a quiet machine).

Pre-release for v2.2.0.

🤖 Generated with Claude Code