Apply batched session and browser configuration plus optional navigation

Optional multipart parts apply configuration while Chromium stays stopped once (policy, flags, extensions, profile archive, optional display sizing), then Chromium is started exactly once and DevTools readiness is awaited. Optional start_url dispatches a best-effort navigation after readiness without waiting for page load. Bare hosts are normalized to https://. Omit any part you do not need. At least one actionable part must be present. Required configuration steps run in this order: policies, extensions, display, flags, then profile archive. Chromium is started only once at the end. The endpoint is not transactional: if a later required step fails, earlier successful side effects may remain on the instance while the non-2xx error propagates. Prefer this over separate /chromium/* and /display calls when multiple restart-triggering steps apply in one session configure.