Optional consumer frame-rate cap for the live persistent interactive loop (feature 121, FR-001/FR-002). `None` keeps the default 60 FPS — the exact pre-feature-121 behaviour. `Some n` (n > 0) bounds BOTH the update and the present cadence of the native event loop, so a host without a blocking compositor does not free-run the present loop; `Some n` with n <= 0 is rejected at startup validation. Ignored by the offscreen/evidence (`runBounded`) path, which does not use the persistent event loop.

Live present mechanism. Defaults to `ViewerPresentMode.DirectToSwapchain` (feature 119) — the readback-free direct present on the OpenGL backend (the scene is drawn straight onto the default framebuffer and presented by the toolkit buffer swap, no per-frame GPU→CPU readback). Set to `ViewerPresentMode.OffscreenReadback` only for evidence/screenshot capture that needs a readback. (Feature 120, FR-016: corrects stale feature-118 text that named `OffscreenReadback` as the default; the shipped default lives at `Viewer.defaultConfiguration`, which uses `DirectToSwapchain` — the docstring is brought into agreement with it.)