ef target architecture primer

Load this before any non-trivial ef work. Full detail: misc/docs/ef_design_notes.md, misc/docs/ef_use_cases.md, .claude/CLAUDE.md.

What ef is becoming

A facade for boilerplate-less semantic-search / RAG / corpus-indexing. Light case in one line (a list of strings → search); heavy case (huge corpora, many segmentations/embedders, varied sources/targets) with the same facade. Uses vd for vector storage. ef is not RAG — no answer synthesis.

ef started as a prototype of an embedding visualization pipeline (segment → embed → planarize → cluster). The 8-phase refactor (ef#3–#18) made search/RAG/corpus-indexing the primary surface and demoted visualization to a secondary "explore the corpus" layer (L5, ef/exploration.py). The refactor is complete. ef is free to change — no users.