docs: record the true-decomposition pattern (ChatOverlayLayer) + handoff refresh
ai.md: Key patterns gains the 'true decomposition beats partial-shuffling' rule + the glue-vs-component diagnostic (count OnPropertyChanged/SetProperty; extract only zero/low-glue cohesive state blobs like chat's; leave binding glue over already-extracted services alone; line count is a guideline not a goal). ViewModels table row notes Chat.cs is now a thin facade over ChatOverlayLayer. HANDOFF: everything through Commit G is pushed (origin/main=85893ea); the remaining big decomposition is the scene-graph segment, flagged as design-needed (not an unsupervised peel).
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