createLatencyProbe

Description#

Measures how long the previous busy window lasted, from a boolean-busy source.

The duration is a wall-clock sample taken at the busy edge - not derivable from signals alone - so the probe writes lastDuration imperatively from an effect. This is a measurement side effect: a distinct write-in-effect pattern, not the same mechanism as createVisibilityGate (which defers its visible write via setTimeout) nor the CngxAsyncContainer exception. It is loop-safe by construction: the effect tracks only busy(), reads the clock in untracked(), and never reads lastDuration back.

Aggregate semantics: the probe measures the busy-envelope. On a false->true edge it stamps startedAt; on the matching true->false edge it records lastDuration = clock() - startedAt. Across concurrent operations behind one boolean source (e.g. CngxAsyncRegistry.isAnythingLoading) that is the whole in-flight window, not any single operation.

Observation invariant: the clock is stamped at the busy transition at effect-flush granularity, independent of whether or when lastDuration is read. A busy window that opens and closes within a single synchronous tick, before change detection flushes the effect, is not measured - real registry transitions cross async boundaries (macrotasks), so the effect flushes between them.

Must be called in an injection context: the internal effect auto-cleans on destroy, mirroring createVisibilityGate.