attest/adjustable-value flags adjustable controls — sliders, steppers — that
respond to the increase or decrease action but expose no current value. A
screen reader lets the user change the setting yet cannot announce what it is,
and a visible label ("Volume") is not the value. This is the Value leg of
Name, Role, Value that the existing rules did not cover.

Deterministic, tree-only, so no Flutter-side change: the builder already
captures increase/decrease actions. Ships with rule fixtures, ten corpus cases
(precision and recall 1.0), and its entry in the coverage matrix under 4.1.2.
Fourteen bundled rules; corpus at 128 cases.

Releases attest 1.5.0 and attest_flutter 1.1.1 (which just requires the new
core and refreshes its docs); attest_cli is unaffected.