system traces: migrate remaining browser sampler traces to trackers #60

Memory, connection, orientation and device memory still come from the legacy browser sampler (packages/browser/src/system-metrics.ts). Only battery was migrated in PR #55.

Slices: tracker per signal (connection with onLine-first mapper, orientation, memory, device memory), a shared PageLifecycleTracker, then delete the legacy samplers (browser and node system-metrics.ts / guarded variants) and the duplicated guard helpers. Then new traces (ST1+) and Electron trackers (ST6). See docs/design/system-traces-expansion.md §14.

Deferred from PR #55 (trace producers over shared trackers).

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