chore(stammbaum): add /api/network capture script + canonical fixture (#361)

Local-only developer utility that authenticates against the running backend,
captures the current /api/network snapshot, and writes it to
src/lib/person/genealogy/__fixtures__/stammbaum.json. Sanity gates exit
non-zero on a vacuous capture (< 50 nodes, < 5 generations, 0 SPOUSE_OF
edges). Fixture and script land together so the fixture is reproducible from
the script that generated it.

Captured snapshot: 62 nodes, 43 edges, 28 SPOUSE_OF (0 with fromYear),
generations G0-G4. Albert de Gruyter is the canonical multi-spouse case with
4 marriages.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/lib/person/genealogy/__fixtures__/README.md

@@ -0,0 +1,25 @@
+# `__fixtures__/`
+
+Pinned real-data fixtures used by Stammbaum layout tests.
+
+## `stammbaum.json`
+
+Snapshot of `GET /api/network` against the canonical Familienarchiv dataset.
+Captured by `frontend/scripts/capture-network-fixture.mjs`.
+
+### Lifecycle
+
+The fixture is **pinned**, not auto-tracked. Tests assert _structural_
+properties (e.g. "a person with ≥ 2 spouses exists") rather than identity
+("Albert has exactly 4 spouses"), so the fixture survives data growth without
+mechanical edits.
+
+Re-capture and update the affected tests in a single intentional commit when a
+new structural case appears in the production graph (new edge type, new
+marriage configuration, new generation range).
+
+### PII
+
+The repository is private and the fixture contains real family names. If the
+repository ever opens, scrubbing is a one-shot migration commit, not a
+permanent test-authoring constraint.