Let the page decide whether we are on the start screen

[PatrickSt1991]

His fourth report, and the best one:

when i open site that had images turned on previously from favourites and turn images off it turns off images globally instead for that site … if visit website by typing url turning images on/off changes setting only for that site

One wrong bit

_atHome was set by ShowHome and cleared by Navigate — that is, by whoever asked for a page. That misses every navigation this app does not perform itself, and opening a favourite is one of them: a tile is an ordinary link (the whole reason the start screen is a page rather than a native screen), so the engine follows it and nothing tells the app it left.

The browser then sat on Instagram still believing it was showing its own start screen. Everything keyed to that went wrong in a different way:

• the images and identity switches wrote the browser-wide setting instead of the site's — because "no site" is exactly what the start screen is, so PageUrl() returned null and SiteRules.SetImages(null, …) correctly declined;
• key 8 looked for a tile under the pointer and found none;
• the address bar said start screen over a page that plainly was not it;
• and turning images back on flashed them for one frame before the reload put the site's own rule back. That last one is the symptom that proves the diagnosis: it is the global setting and the site rule disagreeing out loud, exactly as the code would predict.

His control case — typing the URL — worked, because Navigate() is the one path that did clear the flag.

The fix, and the rule behind it

NoteShowing is called at every load boundary and sets the flag from Store.IsGenerated, so a navigation nobody in this app initiated still moves it. ShowHome and Navigate still set it optimistically (the bar should not lag a press) but are no longer the authority.

State that describes the page must be derived from the page, not from who asked for it.

This had been wrong since the start screen existed. It cost nothing until #74 gave the app something important to decide with it — the usual shape: the bug ships years before the feature that makes it reachable.

Checks

./build.sh all (five packages, 0 warnings) and tools/startpage/run.sh green. No src/common change this time — the decision it now defers to, Store.IsGenerated, is the one the start-page harness has covered since #53.