PERF.56 Apps: the shell costs 154 to 257 ms of main thread per start, and three document-wide MutationObservers fire 100+ times a second while drawing

Measured per app: app-phone.js 17 to 56 ms running the phone layout on a desktop viewport; app-boot.js 10 to 73 ms with a body-subtree MutationObserver whose say() rewrites the DOM on every mutation (app-boot.js:82-83) and never settles while the pointer moves (141 to 221 boot-step rewrites in 3 s of drawing); three observers cascade (app-shell.js:477-482, app-phone.js:421 and :483; CadCraft 360 observer callbacks in 3 s of drawing); app-settings.js 15 to 65 ms; app-devices-ui.js 26 ms. Shell bytes: 1.35 MB fetched, about 780 KB never run (app-addons-account.js 59 KB when signed out, calc.js, ai-*.js, app-devices-ui.js, app-propgrid.js). Change: app-phone returns after the matchMedia check on wide pointer-fine screens and attaches its observers only when cc-phone is on; app-boot ignores mutations inside output-like elements and disconnects on settle; app-shell observes dialog additions only (or an attributeFilter without childList on the whole tree); dynamic-import addons-account, calc, ai-*, devices-ui on first use.