max / quasi

Give Field's last five members builders, 0.101.1 `placeholder`, `max_length`, `min`, `max` and `extended` were public fields with no builder method, so the only way to set one was to assign it. That is `Cells::current`'s defect on the vocabulary's largest struct, and the measured cost is 66 sites across the three shape trees breaking out of a builder chain to write one of them, 54 of those for `placeholder` alone. It also made those five unsayable in a declared description. The form resolves an attribute to a builder method's name, so a member with no builder cannot be reached at all -- probes against goingson's events.rs, settings/email.rs and tasks.rs and audiofiles' classifier.rs each failed on it independently. `within(min, max)` sets the pair, on `Cells::ticking`'s grounds: a bound only means anything against the other one, and `Field::range` already takes both together for that reason. `at_least` and `at_most` set one end, because an open end is a real answer rather than a missing one, which `Field::interval`'s own doc argues. `extended()` takes no argument, matching the form's presence-flag rule. Additive, so a patch: every consumer requirement of "0.101" still resolves. 191 lib tests green.