docs: add SVD axis labels design spec

docs/superpowers/specs/2026-04-05-svd-axis-labels-design.md

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+# SVD Axis Labels: Enforce Canonical Left/Right Orientation
+
+## Problem
+
+SVD axis labels do not consistently reflect the repository's political convention: right-wing parties (PVV, FVD, JA21, SGP) must appear on the RIGHT side of all axes in visualizations. Current code uses inconsistent inline party sets across files (`svd_labels.py` has 9 right parties, `political_axis.py` has 6), and no automated validation exists to enforce the convention.
+
+## Goals
+
+- Guarantee that canonical right-wing parties (PVV, FVD, JA21, SGP) appear on the RIGHT side of all SVD axes.
+- Centralize canonical party sets in a single source of truth.
+- Automatically flip axis orientation when canonical parties appear on the wrong side.
+- Add unit tests and CI validation to prevent regressions.
+- Maintain backward compatibility for existing callers.
+
+## Architecture
+
+### New Module: `analysis/config.py`
+
+```python
+"""Canonical political party sets for axis orientation validation."""
+
+CANONICAL_RIGHT = frozenset({"PVV", "FVD", "JA21", "SGP"})
+CANONICAL_LEFT = frozenset({
+    "SP", "PvdA", "GL", "GroenLinks", "GroenLinks-PvdA",
+    "DENK", "PvdD", "Volt"
+})
+```
+
+### Updated Module: `analysis/svd_labels.py`
+
+- Import canonical sets from `analysis.config`.
+- Replace inline `RIGHT_PARTIES` / `LEFT_PARTIES` with config values.
+- Export backward-compat aliases:
+  ```python
+  from analysis.config import CANONICAL_RIGHT, CANONICAL_LEFT
+  RIGHT_PARTIES = CANONICAL_RIGHT
+  LEFT_PARTIES = CANONICAL_LEFT
+  ```
+- Update `compute_flip_direction()` to use `CANONICAL_RIGHT` for flip decisions.
+
+### Data Flow
+
+1. Compute raw component scores/centroids (existing logic).
+2. Project canonical right party vectors onto the component.
+3. Determine where canonical right parties lie along the component axis.
+4. If majority of `CANONICAL_RIGHT` items are on the left side, set `flip = True`.
+5. When `flip` is True, swap/negate the axis and flip label semantics before returning to callers.
+6. Numerical PCs remain intact; only orientation + labels change at presentation boundary.
+
+### Error Handling & Fallbacks
+
+- **No canonical parties present**: Log warning, fall back to existing behavior (no flip), optionally surface UI notice.
+- **Ambiguous placement**: Use majority rule with deterministic tie-breaking (tie → do not flip, log warning).
+- **Backward compatibility**: Preserve public functions (`get_svd_label`, `get_fallback_labels`, `compute_flip_direction`); only orientation may flip.
+
+## Testing Strategy
+
+### Unit Tests (`tests/test_svd_labels.py`)
+
+- **Synthetic flip test**: Create synthetic vectors where canonical right parties have negative component values; assert `compute_flip_direction` returns `True` and labels map correctly.
+- **Synthetic no-flip test**: Canonical right parties on positive side; assert `compute_flip_direction` returns `False`.
+- **Real window test**: Load a representative window and assert every item from `CANONICAL_RIGHT` appears on the right side after flip logic.
+- **Backward compat test**: Existing tests in `test_svd_labels.py` continue to pass after import swap.
+
+### CI Validation
+
+- Add pytest test group enforcing the rule:
+  ```bash
+  pytest -q tests/test_svd_labels.py::test_canonical_right_on_right
+  ```
+- Test loads at least one representative window and asserts PVV/FVD/JA21/SGP appear on the right side of the principal political axis.
+- Prevents regressions in PR pipeline.
+
+## Migration Plan
+
+1. Create `analysis/config.py` with canonical sets.
+2. Update `analysis/svd_labels.py` imports and export aliases.
+3. Update `compute_flip_direction` to use canonical sets.
+4. Add/update unit tests in `tests/test_svd_labels.py`.
+5. Run full test suite to verify backward compatibility.
+6. Commit changes locally (no push until user approves).
+
+### Out of Scope (Follow-up)
+
+- `analysis/political_axis.py` currently uses different party sets for PCA centroid orientation. Recommended follow-up: align with `config.py` in separate PR.
+- UI notice for missing canonical parties (non-blocking enhancement).
+
+## Success Criteria
+
+- `CANONICAL_RIGHT` and `CANONICAL_LEFT` defined in `analysis/config.py`.
+- `svd_labels.py` imports from config; exports aliases for backward compat.
+- All existing tests pass after changes.
+- New test `test_canonical_right_on_right` passes.
+- Running `pytest tests/test_svd_labels.py -q` shows no regressions.
+- Right-wing parties (PVV, FVD, JA21, SGP) consistently appear on the RIGHT side of all axes in visualizations.
+
+## Related Files
+
+- `analysis/config.py` (new)
+- `analysis/svd_labels.py` (update)
+- `tests/test_svd_labels.py` (update)
+- `docs/solutions/best-practices/svd-labels-voting-patterns-not-semantics.md` (existing convention doc)
+- `AGENTS.md` (convention reference)