Move-only: tests to tests.rs, then the 6.8k-line lib.rs into 12 concept modules (buffers, editing, fuzzy, markdown, motions, palette, prefs, render, search, snippets, undo, visual). impl Editor is split across them; moved private items become pub(crate) and the public API is preserved via root re-exports. No logic changes; 221 tests and the firmware build pass unchanged.