Pivot from Onshape to FreeCAD addon

[tophcodes]

Pivots Space Elevator away from the Tauri+Vue GUI / Onshape browser extension toward a FreeCAD-native Python workbench backed by an optional Rust daemon for the SpaceMouse Enterprise LCD. Spec: docs/superpowers/specs/2026-05-26-freecad-first-pivot-design.md. Plan: docs/superpowers/plans/2026-05-26-freecad-first-pivot.md.

22 commits across three deliverables:

• Cleanup — drops the space-elevator/ Vue+Tauri subdir, establishes a Cargo workspace, slims devenv.nix (no more webkit/bun, adds Python+pytest), rewrites the top-level README.
• space-elevatord (new Rust binary crate) — tokio-based Unix socket server at $XDG_RUNTIME_DIR/space-elevator.sock, newline-delimited JSON IPC, dispatches ping / lcd_clear / lcd_display_image / lcd_display_svg. Reuses spnav::lcd for hardware. 5 unit tests.
• freecad-addon (new Python workbench) — ctypes bindings for libspnav, QSocketNotifier-driven event pump, camera navigation via Coin3D, button-to-command bindings stored in App.ParamGet, optional LCD label push to the daemon, Preferences page and a button-bindings dialog. 14 unit tests.

Motion path (libspnav -> ctypes -> QSocketNotifier -> camera) and LCD path (addon -> Unix socket -> daemon -> HID) are fully independent; either can fail without affecting the other. Daemon absence degrades silently.

Note for the future: spec's IPC surface (set_buttons/set_status) doesn't match the as-built protocol (lcd_display_svg); the plan documents this scope cut. Spec to be updated post-merge.

Manual verification

Subagent implementation can't drive hardware or FreeCAD's UI, so the following hasn't been exercised yet and gates the merge:

• cargo run -p space-elevatord starts cleanly
• ping round-trip via nc -U returns {"ok":true} (actual reply is the superset {"v":1,"id":N,"ok":true}; request must be {"v":1,"id":N,"cmd":"ping"})
• lcd_display_image with a 640x150 PNG renders on the SpaceMouse Enterprise LCD (will be garbled until spnav/src/lcd.rs packet header TODOs are resolved — pre-existing)
• lcd_display_svg with a small SVG renders
• Symlink freecad-addon/ into ~/.local/share/FreeCAD/Mod/SpaceElevator and launch FreeCAD
• "Space Elevator" appears in the workbench dropdown
• Activating logs Space Elevator: event pump started
• SpaceMouse motion pans/rotates the active 3D view
• Bind button_0 to Std_New via the Python console, press button 0 — new document opens
• Space Elevator -> Button bindings... menu opens the dialog; saving persists
• Edit -> Preferences -> Space Elevator shows axis-scale and LCD settings; changes persist
• With daemon running and one binding set, switching to the workbench renders labels on the LCD
• With daemon stopped, workbench activation still works; no popup, no Report View error

Follow-ups (file as issues post-merge)

• libspnav singleton isn't idempotent across deactivate/activate. Second activation will fail silently because spnav_open() on an open connection returns -1. Track _opened state on _Lib. See freecad-addon/space_elevator/libspnav.py:93 and event_pump.py:27.
• Spec's IPC surface vs. as-built — update spec or move SVG rendering into the daemon.
• LCD labels only push on activation, not on bindings save. Call _push_labels from BindingsDialog._save or document the contract.
• _short_name strips the namespace blindly; cross-workbench command collisions produce identical labels.
• LcdClient not thread-safe — fine on the Qt main thread, but should be documented or guarded.
• Pre-existing: spnav/src/lcd.rs packet header TODOs — daemon LCD output won't be reliable until resolved. Out of scope for this PR.

Closes #1.