Derive masonry columns from width + instrument rotation reflow

Created by: jedmund

Summary

• Replace the size-class + AppStorage column-count scheme with a width-driven bucket (2/3/4/5) derived from each ScrollView's live width via .onGeometryChange with an Int transform, so reflow fires only on bucket flips — not on every width pixel during a rotation animation. iPhone is locked to portrait since the width-driven scheme has no need for an iPhone-landscape bucket.
• Add OSSignposter + companion Logger probes (masonry-distribute, column-bucket-change, cell-onAppear / cell-onDisappear, pixivimage-load, cellframe-write, appstorage-first-read) under subsystem com.attractor.pixillate, category rotation, so Instruments' Points-of-Interest track surfaces the moving pieces of a column-bucket flip next time reflow regresses. Probes are effectively free when Instruments isn't recording.

Cold-install + rotate on a physical iPad (iPad Pro 11", iOS 26.x) is now smooth — no squished first frame. Keeping the probes in place so the next regression is one trace away from a diagnosis.

Test plan

• Builds for iPad Pro 13" simulator (iOS 26.2, Debug).
• Cold install on physical iPad → rotate once → no freeze, no squished frame.
• Verify Home / Tag / Profile / RelatedIllustsSection all reflow cleanly on rotation.
• Verify detail overlay's nested RelatedIllustsSection also repaginates on rotation.
• Spot-check signposts: log stream --predicate 'subsystem == "com.attractor.pixillate" && category == "rotation"' with the iPad tethered.
• Verify the "Grid" section no longer appears in Settings.

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