Second-review fix: explicitInputFlagName judged usability with an
unconditional TrimSpace while rawValue only trims when Trim is set. For
a non-Trim flag a whitespace main value is usable and shadows a changed
alias; the resolver could then rewrite the shadowed alias (and fail on
its @path) while the fallback chain still read the main value. Mirror
rawValue's usable() exactly and pin the shadow case with a regression
test whose alias path does not exist.