Bracket property writes catch up, and objects build leaner

Two more speed wins in the corner of the engine most real code lives in. Last release taught the engine to READ a property fast when it is written with a constant bracket key - `thing['name']`, the form minifiers emit for most of the web. This release does the same for WRITING one: `thing['name'] = value` now runs about twice as fast, where it had been left at plain-interpreter speed. It stays exact by construction - the moment the value being assigned does anything unusual (a function call, another object), the write quietly takes the careful path, so nothing changes but the speed. Second, building objects. Every time code makes an object like {x: 1, y: 2}, the engine was doing bookkeeping to guard against duplicate keys - useful in the rare case, wasted work in the overwhelmingly common one where the keys are all different. It now skips that scan when the keys are plainly distinct, so building a wide object runs about 15 percent faster. None of it changes a result: both are proven equal to the plain interpreter by a differential fuzzer, and the whole engine gate holds at 6375 of 6375.