SystemMetricsValueStream used to dispatch on its ``self._metric`` string
across three independent if/elif chains (audit finding M5):
* priming in ``start()`` (cpu_percent seed, initial network counter);
* raw reading in ``_read_metric_psutil`` plus ``_read_metric_fallback``;
* normalisation in ``_normalize`` (percent / min-max range / max-rate).
Adding a new metric meant editing all three chains plus the Android
fallback — and forgetting one branch made the metric silently return 0.
Lift each per-metric concern into a free function and register them as a
``MetricSpec(name, read_psutil, read_fallback, normalize, prime)`` in a
new ``core.processing.metric_readers`` module. Shared normalisers
(``_norm_percent`` / ``_norm_range`` / ``_norm_rate`` / ``_zero``) live
once. The stream's ``start()`` / ``_read_metric()`` / ``_normalize()``
collapse to a single registry lookup + delegation.
The stream still owns its mutable state (``_disk_path``,
``_sensor_label``, ``_gpu_unavailable``, ``_prev_net_bytes``,
``_prev_net_time``, etc.) — readers operate on the stream by
parameter, not by inheritance, so the kitchen-sink class shrinks by
~140 lines without losing the per-stream cadence bookkeeping. Each
spec function's docstring documents which fields it reads or mutates.
Tests: 16 new tests cover the 10-metric coverage set, callable shape
of every spec field, the three normaliser primitives' clamping +
divide-by-zero behaviour, prime-hook presence (only the three metrics
that need a baseline: cpu_load + network_rx + network_tx), and
fallback-path expectations (desktop-only sensors -> _zero, cpu/ram ->
real MetricsProvider).
754 existing core / storage / api tests stay green; ruff clean.
alexei.dolgolyov
9f3f346543