[wip] Feature/measure v2 scan nd

docs/changes/newsfragments/+measure_v2_scan_nd.new

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+Added ``scan_nd`` to ``qcodes.measure_v2`` for nested multi-dimensional
+sweeps. Sweeps are passed outermost-first
+(``scan_nd(outer, middle, inner, measured=[...])``); the plan-builder reads
+every ``measured`` parameter at each innermost setpoint and emits one
+dataset row per innermost point. Per-sweep ``delay`` and ``get_after_set``
+are honored. On exit (success, error, or cancel), every swept parameter
+is set to ``0.0`` — extending the ``scan_1d`` cancel-safety contract to
+N dimensions. The convenience entrypoint ``qc.measure_v2.scan(*sweeps,
+measure=[...])`` now accepts any number of sweeps and dispatches to
+``scan_1d`` or ``scan_nd`` accordingly.

docs/changes/newsfragments/8116.new

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+Added experimental ``qcodes.measure_v2`` package — a parallel measurement API
+that decouples plan description from execution. Plans are Python generators
+yielding typed messages (``Set``, ``Read``, ``Sleep``, ``Emit``, ``OpenRun``,
+``CloseRun``); an engine drives them on a dedicated worker thread and
+publishes events to pluggable sinks. Enables non-blocking measurement
+submission, mid-flight cancellation with guaranteed cleanup via plan
+``try/finally`` blocks, and live data consumption via custom sinks.
+The public surface is ``qc.measure_v2.scan(...)`` and the engine /
+``RunHandle`` types for advanced use; the SQLite sink reuses the existing
+``DataSaver`` infrastructure. This package is experimental and its API
+may change in subsequent releases. See ``src/qcodes/measure_v2/DESIGN.md``
+for the architecture document.