[frectrium]

What

Adds docs/design/architecture.md: a Magic Pocket diagram and a matching Satchel diagram, each with numbered put and get paths, a one-line role per component with source IDs, and a table of what Satchel keeps and simplifies.

Closes #11

Why

Objective 2 in the brief, and the architecture section of the Week 2 report (#18). The solution section (#50) is waiting on this diagram.

How to check it

• The diagrams are Mermaid, so open the file on GitHub to see them rendered.
• Section 1.4: every Magic Pocket role has a source ID from the architecture notes ([Research] Extract Magic Pocket's architecture from sources #8).
• Section 2.4: every simplification points to its Design Decision Log entry.
• @VyasGuru, check the component order matches your solution walk-through in docs(report): explain how Magic Pocket works #50.

Merge order

Merge after the Design Decision Log (#55) and the exercise plan (#56), since the Satchel half follows them.

Checklist

• cargo fmt, cargo clippy and cargo test pass locally (code PRs)
• Tests added or updated for the change (code PRs)
• New sources are added to docs/sources/README.md (research PRs)
• Design decisions are recorded in the Design Decision Log
• Docs and README are updated if behaviour or setup changed