Second windows work, and loops with real helpers get compiled

Two very different fixes in one release. First, the embarrassing one: opening a second window (Cmd+N, or restoring windows from a previous session) could come up as a dark, empty shell on macOS. The page was actually there and running; a one-line bug in how the window laid out its views was hiding the window's own interface. That is fixed, second windows now render exactly like the first, and while we were in there we reworked how a previous session comes back: Floati no longer piles your old windows onto the screen at launch, and instead of a button on the New Tab page there is now a quiet line on the History page offering to reopen them when you actually want them. Windows that were just an empty New Tab are not offered at all, and new windows skip the startup splash and open straight into the browser. Second, the engine: last release taught hot loops to absorb tiny one-line helper functions; this release extends that to real ones, helpers with their own local variables and several steps of work. A loop calling such a helper a million times runs about three and a half times faster, a shape that previously could not be compiled at all. We also completed the groundwork experiment for the next big leap, compiling hot loops all the way to machine code; it is not switched on in what you are running, but the results (twenty times faster again on simple numeric loops, with the full engine test suite passing identically) confirmed the direction.