Add the orbit-for-all-exercises handoff plan

Claude-Session: https://claude.ai/code/session_01LEoff8bXGBS83tK1c55Mf7

Exercise Library/TODO-orbit.md

@@ -0,0 +1,90 @@
+# Handoff: complete camera rotation (slow orbit) for all exercises
+
+Repo: `~/Documents/indie/Workouts`. Read `Exercise Library/SYSTEM.md` first
+(format spec + visual language), then `Exercise Library/kinematics.py` and
+`render.py` (reference renderer), and `Workouts/ExerciseFigure/`
+(`MotionSolver.swift`, `ExerciseFigureView.swift`, `ExerciseMotion.swift` —
+the Swift port, kept in lockstep with Python; shared by the iOS and watch
+targets).
+
+## Current state
+
+The figure rig is an anatomical 3D skeleton with a per-exercise orthographic
+camera (`camera: {yaw, pitch}`, default pitch 10°). In the app,
+`FigureAnimation` (ExerciseFigureView.swift) slowly orbits the camera (360°
+per 24 s, `orbitPeriod`) — but only when `props.isEmpty && hide.isEmpty`,
+i.e. the 8 bodyweight moves. The 14 machine exercises stay at their authored
+view because their equipment can't rotate. The goal: **every exercise
+orbits.**
+
+## What blocks it, in order of work
+
+1. **`scene` props are 2D canvas billboards** (seats, masts, benches —
+   `line`/`rect`/`circle` in canvas coords). They need world-space 3D form.
+   Follow the exercise-mat pattern (`geo["floor"]` in `render.py` /
+   `FigureGeometry.floor` in Swift — a quad built in model space relative to
+   the root anchor, rotated through `Rx(pitch)·Ry(−yaw)`, then projected):
+   extend the scene-shape schema so each shape carries depth — suggest an
+   optional `"z"` (depth plane, default 0) plus `"depth"` (extrusion
+   half-width) so a seat rect becomes a slab and a mast line a post; author
+   full 3D endpoints only where needed. Convert authored canvas coords to
+   model space by subtracting the motion's root anchor (same as the mat).
+   Re-author all 14 machines' props minimally — schematic silhouettes, not
+   scale models; check each at yaw 0 (must look identical to today), ±45°,
+   and 90°.
+2. **`cable`** `from` anchors and **`bar`** fixed world angles are
+   authored-view constructs: make both world-space (the anchor gets a depth,
+   the bar becomes a segment defined in the authored view and rotated like
+   the mat — it must stay welded to the hands).
+3. **`roller`** and **`pad`/`dumbbell`** perpendiculars are computed in
+   screen space per frame — under orbit compute them in the flat authored
+   view, then rotate (the resolve-then-rotate invariant below).
+4. **`hide` lists** name limbs occluded in one authored view — wrong under
+   orbit. Only Side Plank uses one; re-author it with genuine 3D (root
+   `roll`, real abduction) instead, then delete the `hide` mechanism if
+   nothing else uses it.
+5. Relax the `orbits` gate in `FigureAnimation` to always-on once 1–4 hold
+   up visually.
+
+## Invariants — do not break
+
+- **Resolve flat, then rotate.** IK pins and prop attachment logic operate
+  in the *authored* view (yaw as authored, pitch 0); the *posed result* is
+  rotated for presentation. Never re-solve pins or recompute screen-space
+  perpendiculars in the rotated view — that class of bug ("hands stuck
+  mid-orbit") was already fixed twice.
+- **Both renderers in lockstep.** Every geometry change lands in
+  `render.py`/`kinematics.py` *and* the Swift port, same conventions and
+  constants. `WorkoutsTests/Fixtures/figure-fixtures.json` pins Swift to
+  Python (0.5 px); regenerate it from the Python pipeline whenever projected
+  geometry changes (per exercise/keyframe snapshot: order, shade, spine,
+  head, nose, limb chains, plus tween + orbit samples — extend the orbit
+  samples to a machine exercise once props rotate).
+- **Visual parity at the authored view**: rendering any exercise at its
+  authored yaw must look the same as today. Verify with
+  `python3 render.py --sheet` before/after, and orbit strips
+  (`--orbit <name>`, or a multi-yaw contact sheet) for each converted
+  machine.
+
+## Verification workflow
+
+`python3 render.py` + `--sheet`/`--orbit` and eyeball the PNGs;
+`python3 render.py --export` (verbatim copy into
+`Workouts/Resources/ExerciseMotions/`); `xcodegen generate` if project.yml
+changes; `xcodebuild test -scheme Workouts -destination 'platform=iOS
+Simulator,name=iPhone 17 Pro' -only-testing:WorkoutsTests` (48 tests must
+stay green, plus whatever you add); `xcodebuild build -scheme "Workouts
+Watch App" -destination 'generic/platform=watchOS Simulator'`. Update
+`SYSTEM.md` (props section) and add one end-user CHANGELOG.md entry per the
+app-changelog skill (one sentence, no bullets, top of the July 2026 block —
+e.g. machine form guides now rotate too). Commit per feature with the
+repo's existing message style and push. Delete this file in the final
+commit.
+
+## Judgment calls you're empowered to make
+
+Exact 3D prop schema shape (keep it minimal and hand-authorable),
+per-machine depth values, and whether a couple of visually-degenerate
+machines (e.g. cable stacks) look better with a reduced orbit sweep (±40°
+pendulum instead of full 360°) — if so, make sweep style a per-motion
+`camera` option with full-360 the default.