docs: document Canopy design and Cellule architecture

[forhappy]

Canopy needs one design reference that connects its repository model, Cellule integration, protocol flows and operational boundaries. Add root DESIGN.md with goals, implementation status, terminology and invariants, component responsibilities, durable command/receipt semantics, routing, push and replay, reads/LFS, recovery, collaboration, resource ownership, security, failure handling, tradeoffs and verification obligations.

Provide nine source-linked architecture diagrams as standalone SVGs and 2x PNGs, with an offline gallery and reproducible Python tooling. Keep the diagram-folder README as an export/regeneration index. The gallery selects the matching sections from the root design, renders their tables, and rebases local links. Both documentation indexes point to the root reference.

The design records the inspected Canopy revision and exact Cellule dependency pin, separating the active SQL serving path from incomplete immutable-catalog production integration and proposed composed capabilities. Sequence diagrams keep activation bars behind dashed lifelines and message arrows. The Repository Cell–S3 connector descends from a bottom anchor before turning.

Validation:

• Checked 283 local links and anchors across the design, documentation indexes, diagram index and gallery; all passed.
• Parsed all nine standalone/embedded SVGs, checked 2x PNG dimensions, unique gallery IDs, source-map paths, Python syntax and five generated design tables.
• Reviewed diagram exports and the gallery, including the expanded design prose/table rendering, sequence layering and S3 connector.
• git diff --check passed. Rust tests were not run because changes are documentation and diagram tooling only.