Serve a placeholder icon instead of linking one that is not there

[seenu-k]

Every page the engine renders linked /favicon.png and /icons/Icon-192.png unconditionally. Those are static assets from the game's own public/, which a fresh scaffold ships holding a single .gitkeep, so until a Flutter web build landed there the browser tab was blank and all four manifest icons 404ed.

An Android-only game never runs build:web, so it stayed that way permanently — even though the download page's hero already had a fallback for exactly this case. The asymmetry is what this fixes.

What changes

No icons in public/	Icons in public/
<link rel="icon" type="image/svg+xml" href="/_eigen/icon/v1/mark.svg">	<link rel="icon" href="/favicon.png"> + apple-touch
manifest lists one scalable icon	manifest lists the four PNGs
hero shows the inline mark	hero shows Icon-192.png

The mark is served by the worker, drawn in site.primaryColor, and switches its neutral stroke for a dark tab strip. Apple's touch icon is omitted rather than pointed at a missing file, since it has no SVG support; iOS then derives one from the page, which beats a broken request.

It is a placeholder, not a default. The moment favicon.png exists the engine links the game's own icons everywhere and stops serving its own, so the override needs no configuration.

The probe split

hasWebBuild asks for index.html and still gates "Play on the web". The new hasAppIcons asks for favicon.png and gates every icon. These were one question, on the stated grounds that flutter build web emits both together: true for a game that ships on the web, wrong for the case worth supporting.

hasAppIcons checks the response's content type as well as its status, because the scaffold's single-page-application fallback answers 200 OK with index.html for any missing asset once a web build exists. Status alone would read that as an icon and link a web page where an image belongs.

Notes

• No run_worker_first change needed: under assets_navigation_prefers_asset_serving only navigation requests bypass the Worker, and an icon fetch is a subresource. This is the same route mechanism the bundled fonts already use.
• The manifest moves from a 24h to a 1h cache. It used to be a constant and now depends on what is in public/, so a day-old copy would hide the very deploy that adds icons.
• The placeholder is deliberately not immutable: its bytes carry site.primaryColor.

Verification

174 tests pass. Four are new, and they close the branch the suite previously carried as KNOWN GAP: the other side of this branch is untested — including an Android-only game with icons and no web build, and the SPA-fallback false positive.

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