The label195 and label562 probe blocks each clamped the probed of_duration to
10s, wrote it to the stream's _.dur file and raised $rSegmentTime -- identical
seven-line cores. Extract them into a shared persistSegmentDuration() helper
returning [clamped probe, updated segment time], with unit tests (temp
STREAMS_PATH). No control-flow change; behaviour preserved.
Verified: phpstan level 5 green, phpunit 11 monitor tests green, make gates green.