Add multi-page atlas composition for Cachet slice 2

[waywardmonkeys]

This PR turns the atlas path from a crate sketch into a coherent composed flow.

It adds multi-page rect-atlas support in cachet_storage, class-based page routing incachet_atlas, and an AtlasCache controller that wires residency, routing, allocation, rollback, and diagnostics together without collapsing the crate boundaries. It also deepens the glyph-cache story with a multi-page churn demo, cross-crate integration coverage, and clearer public docs.

Highlights:

• add RectAtlasSet, AtlasPageId, and paged atlas slots/stats
• add AtlasPageRouter for class-to-compatible-page routing
• add AtlasCache as the intended queue/process/resolve atlas composition path
• support rollback when residency admission succeeds but physical placement fails
• expose atlas controller stats and page-local diagnostics
• enforce stable atlas metadata per logical key at queue time
• add integration and regression tests for multi-page reuse and contradictory duplicate requests