Decide target tree on paper before moving anything

Question

Per the conventions (§14 Applying this to an existing repository):

1. Inventory. Done via #2528 and #2529.
2. Decide the target tree on paper before moving anything, and record why in docs/explanation/decisions.md.
3. Move with git mv, in one commit that contains only moves. A commit mixing moves and edits is unreviewable.
4. Fix internal references — relative links, build scripts, imports, CI paths — in a second commit.
5. Re-run the build and the tests against the new layout.
6. Rewrite the entry README last, once the tree it describes has stopped moving.

Decision needed: Create the target tree diagram and decision record before any moves happen. This is a planning/grilling task.

Label: wayfinder:grilling
Part of #2517
Blocks: #2518, #2519, #2520, #2521, #2522, #2523, #2524, #2525, #2526, #2527, #2530

Review conventions §14 and the inventory work in #2528 and #2529 first. Create the target tree diagram and record the rationale in docs/explanation/decisions.md before any moves occur. Done means the target layout and decision record are agreed and ready to guide the later move work.