Add activity filters

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Custom activity filters could help improve activity detection and make it less prone to weird changes.

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Reviewed for 10.0.0: deferred. Activity recognition has fixed internal thresholds, but a user-facing filter model needs clear semantics and configuration design rather than being added as release scope.

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Sol decision: implement post-v10 together with #113 but keep responsibilities separate. #113 defines which semantic categories may auto-start tracking (ON_FOOT, BICYCLE, VEHICLE); #159 remains internal confidence filtering, evidence fusion, grace periods, and hysteresis. Post-session classification must remain independent of the user auto-start selection.