webview: launchWithSeams passes a caller systemMetricsSampler unfiltered, so producer-owned traces get two writers #158

Found by the Opus review of #155 (which fixed the same bug in packages/browser/src/launch.ts, #154). launchWithSeams in packages/webview/src/launch.ts passes the caller's systemMetricsSampler rows to the system traces provider unfiltered, so sampler rows under names a still-running producer owns (battery, charging, route, view, url, capture...) get a second writer. Apply the same rule as #155 (producer-owned names win; the sampler's other rows pass through), with tests; check whether the WebView host-owned suppression (WEBVIEW_HOST_OWNED_TRACES) already covers part of it. Verify against current main first.