Four layout fixes chasing one header down to the pixel

This release keeps walking the encyclopedia header - the worst-scoring part of a page the geometry probe had already narrowed to three faults - and fixes four separate things that made it wrong, each verified against the reference browser on a minimal page before and after. An element whose children are all out-of-flow no longer misreads itself as an inline context, and an inset-anchored absolutely positioned child (the tab-underline every navigation bar draws) is now placed instead of dropped when its container is inline - a fix that also nudged a rendering trophy closer to the reference browser. An icon-only flexbox label now reports a text baseline, so the line holding it stops hanging an extra strut of empty space beneath it. And a custom property that refers to itself - the conditional-inheritance shorthand every design system ships - now resolves the way the standard says, which was the missing piece that made the encyclopedia's menu icon finally size to its correct 20 pixels instead of collapsing to a 10-pixel stub. The reference corpus did not regress on any of its 96 cases and the engine gate holds at 6375 of 6375. Honest note: the header is measurably closer but not yet pixel-exact - the last few pixels are a mix of a delicate line-box calculation and font-metric parity (our normal line height is about a pixel taller than the reference browser's), which is the documented next step.