Fix and Re-measure

Write applyFix(page, fix) producing an improved page spec (non-mutating). Fixes: "server" halves ttfbMs; "hero" sets heroLazy false; "css" empties renderBlocking; "images" quarters imageKB; "js" halves jsKB. Then improved(page, fix) returns { before, after, better } where better is true when the fix actually changed the spec (after differs from before, compare with JSON.stringify).

Difficulty: intermediate