refactor: extract data loading and trajectory logic from explorer.py

- Move trajectory analysis to analysis/trajectory.py (+136 lines)
- Move projection helpers to analysis/projections.py (+128 lines)
- Extract tab-specific data loaders to analysis/tabs/ (8 modules, +133 lines)
- Remove 702 lines from explorer.py (data loading extracted to
  analysis/explorer_data.py and new modules)
- Add axis label fallback tests (tests/test_axis_label_fallback.py)
- Add session docs: brainstorms, ideation, plans, and test-failures

docs/solutions/test-failures/svd-label-tests-after-refactoring.md

@@ -0,0 +1,90 @@
+---
+title: Test assertions failed after extracting SVD_THEMES to separate module
+date: 2026-04-04
+category: docs/solutions/test-failures/
+module: Stemwijzer Data Analysis
+problem_type: test_failure
+component: explorer
+symptoms:
+  - "test_display_label_for_modal" assertion failed with "EU-integratie" not found
+  - "test_get_svd_label_returns_correct_label" assertion failed with "Nationalisme" not found
+  - Tests expected old fallback labels but SVD_THEMES had updated values
+root_cause: test_failure
+resolution_type: test_fix
+severity: medium
+tags: [svd, test-assertions, refactoring, constants]
+affected_files:
+  - tests/test_axis_label_fallback.py
+  - tests/test_svd_labels.py
+  - analysis/config.py
+---
+
+# Test assertions failed after extracting SVD_THEMES to separate module
+
+## Problem
+
+After extracting `SVD_THEMES` constant from `explorer.py` to `analysis/config.py`, tests failed because they hardcoded assertions for old label text.
+
+## Symptoms
+
+- `test_display_label_for_modal`: expected `"EU-integratie" in x_label or "Nationalisme" in x_label`
+- `test_get_svd_label_returns_correct_label`: expected `"EU-integratie" in label1`
+- `test_manifest_loads`: manifest.yaml had `categories:` key instead of `files:`
+
+## What Didn't Work
+
+- Investigating `get_svd_label()` function — it correctly returned values from `SVD_THEMES`
+- Checking import chain — no circular import issues
+- The problem was purely that test assertions hardcoded OLD expected label values
+
+## Solution
+
+Updated test assertions to match the current `SVD_THEMES` values:
+
+**tests/test_axis_label_fallback.py:**
+
+```python
+# Before (incorrect)
+assert "EU-integratie" in x_label or "Nationalisme" in x_label
+assert "Populistisch" in y_label or "Institutioneel" in y_label
+
+# After (correct)
+assert "Rechts kabinetsbeleid" in x_label or "links oppositiebeleid" in x_label
+assert "PVV/FVD-populisme" in y_label or "mainstream-partijen" in y_label
+```
+
+**tests/test_svd_labels.py:**
+
+```python
+# Before (incorrect)
+assert "EU-integratie" in label1 or "Nationalisme" in label1
+
+# After (correct)
+assert "Rechts kabinetsbeleid" in label1 or "links oppositiebeleid" in label1
+```
+
+**Fix manifest.yaml:**
+
+```yaml
+# Before (incorrect)
+categories:
+
+# After (correct)
+files:
+```
+
+## Why This Works
+
+The tests were asserting on hardcoded string values that no longer matched the actual `SVD_THEMES` content. After updating the assertions to check for current label text, tests pass because they correctly verify the actual values returned.
+
+## Prevention
+
+1. **Audit tests when extracting constants** — When extracting constants to separate modules, grep for all test references to those constants and update assertions
+2. **Use flexible assertions** — Prefer `in` checks over exact matches when testing label text, or better yet, import the constant directly in tests and assert equality
+3. **Update manifest tests early** — When changing YAML structure in config files, check for corresponding manifest/schema tests
+
+## Related Issues
+
+- `analysis/config.py` — Contains `SVD_THEMES` (extracted from `explorer.py`)
+- `analysis/svd_labels.py` — Uses `SVD_THEMES` via runtime import from `explorer.py`
+- `docs/solutions/logic-errors/svd-component-labels-mismatch.md` — Background on why SVD labels were updated from semantic to voting-pattern based