chore: scaffold the Duet package skeleton

Initial skeleton for Duet, a two-pane file commander built on dirvish/dired: the package header and entry-command stub, the README, and the test directory. Pre-alpha — no functionality yet.

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+#+TITLE: DUET — Dual-Pane File Commander for Emacs
+
+DUET is a two-pane orthodox file manager (Midnight Commander / FileZilla
+style) built on dirvish/dired. Two dired panes each show any location —
+local or remote — and single-key actions on the file under point use the
+opposite pane as the implied target: copy, move, delete, open. The name is
+recursive: *DUET Unifies Endpoint Trees*.
+
+*Status: pre-alpha.* The interaction model and architecture are designed;
+implementation is staged (below). This repository is the skeleton.
+
+* Why DUET
+
+Two dirvish windows already approximate the basics. DUET's durable value is
+transport reach and a community-extensible backend interface — moving files
+dired cannot, through one consistent two-pane UX.
+
+* The transport quartet
+
+DUET ships four transfer utilities as backends, over TRAMP as the
+Emacs-native substrate. They do not overlap:
+
+- *rsync* — Unix-to-Unix and local. Block-level delta-transfer, zero remote
+  install, faithful Unix metadata. The default for an ssh-reachable host.
+- *rclone* — cloud and object stores plus the long protocol tail (S3, B2,
+  GDrive, WebDAV, ...). The reach rsync lacks.
+- *lftp* — FTP/FTPS/HTTP with mirroring, parallel transfers, queueing.
+- *unison* — bidirectional, conflict-aware reconciliation.
+
+One line: rclone is breadth, rsync is depth.
+
+* Roadmap
+
+- *Stage 1* — core commander: two-pane UX, connection wizard, rsync + TRAMP
+  transport, copy/move/delete/open, the keybinding scheme.
+- *Stage 2* — the backend registry plus rclone and lftp. The differentiator.
+- *Stage 3* — directory diff and bidirectional sync (ediff for files;
+  unison / rclone bisync for trees).
+
+* Extending DUET
+
+Transports are pluggable. A backend registers via =duet-register-backend=
+with:
+
+- a *name*;
+- a *handles* predicate =(lambda (src dst) ...)= returning a numeric score
+  (lower preferred) or nil if it cannot handle the endpoint pair;
+- a *command* builder =(lambda (src dst opts) ...)= returning a process spec;
+- *capability* flags: =:async :resume :bidirectional :progress=.
+
+=duet--transfer-spec= asks every registered backend to score an endpoint
+pair and picks the lowest-cost handler. The built-in backends register
+through this same API, so third parties add transports (unison, git-annex,
+or anything else) without patching core.
+
+* License
+
+GPL-3.0-or-later.