docs(stammbaum): ADR-026 custom viewBox pan/zoom + glossary terms (#692)

Record the reversal of OQ-007 (build custom over the existing viewBox rather
than adopt the panzoom library) and add pan/zoom view-state + fit-to-screen
glossary entries.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/GLOSSARY.md

@@ -125,6 +125,10 @@ _Not to be confused with [parented](#parented-layout)_ — loose is the absence
 
 **canonical fixture** (Stammbaum) — `frontend/src/lib/person/genealogy/__fixtures__/stammbaum.json`, a pinned `/api/network` snapshot used by `buildLayout.test.ts` for structural-property assertions against real data. Captured locally via `frontend/scripts/capture-network-fixture.mjs` with explicit credentials and a localhost backend; never invoked from CI. Sanity-gated by `validateFixture.ts` (≥ 50 nodes / ≥ 5 generations / ≥ 1 SPOUSE_OF edge / ≥ 1 multi-spouse person).
 
+**pan/zoom view state** `[#692]` — the `{ x, y, z }` triple (`PanZoomState` in `frontend/src/lib/person/genealogy/panZoom.ts`) describing the Stammbaum canvas position: `x`/`y` are pan offsets applied to the SVG viewBox centre, `z` is the zoom factor (clamped 0.25–3.0). Mirrored into the shareable URL as `?cx&cy&z` and seeded server-side from those params. See [ADR-026](adr/026-stammbaum-custom-viewbox-pan-zoom.md).
+
+**fit-to-screen** `[user-facing, #692]` — the Stammbaum control (`⤢`) and initial state that frames the whole tree in the viewport. Because the base viewBox already encloses the layout at `z=1`, fit-to-screen is simply the default view `{x:0, y:0, z:1}`.
+
 ---
 
 ## Other Domain Terms

docs/adr/026-stammbaum-custom-viewbox-pan-zoom.md

@@ -0,0 +1,56 @@
+# ADR-026 — Stammbaum pan/zoom is a custom viewBox layer, not a third-party library
+
+**Date:** 2026-05-29
+**Status:** Accepted
+**Issue:** #692 (mobile read path — pan, zoom, fit-to-view); supersedes OQ-007
+**Milestone:** Stammbaum mobile read path
+
+---
+
+## Context
+
+#692 makes `/stammbaum` usable on phones: drag-to-pan, pinch/keyboard/wheel zoom,
+fit-to-screen, recentre-on-person, a shareable URL view state, and an edge-fade
+affordance. During issue grooming, **OQ-007 was resolved to adopt the `panzoom`
+library** (timmywil v4.x) on the team's recommendation, pinned per NFR-MAINT-001.
+
+That recommendation predated a load-bearing implementation detail: `StammbaumTree.svelte`
+already renders zoom by **deriving the SVG `viewBox`** (`w = baseW / z`, centred on the
+layout bounding box, `preserveAspectRatio="xMidYMid meet"`) — not by applying a CSS
+`transform`. The `panzoom` library operates by writing `transform` to a DOM node. Adopting
+it would mean:
+
+- abandoning the proven viewBox derivation and the in-SVG generation gutter (#689), which
+  lives in SVG user-space coordinates and would have to be reconciled with a CSS-transformed
+  parent;
+- re-deriving fit-to-screen, recentre, and the `?cx&cy&z` URL state against the library's
+  transform coordinate system;
+- a client-only lazy import to keep the SSR-rendered tree from touching `window` at module
+  load; and
+- ~8 KB of bundle for behaviour we can express in a few pure functions.
+
+## Decision
+
+**Build pan/zoom as a thin custom layer over the existing viewBox**, with no third-party
+dependency. This reverses OQ-007.
+
+- All geometry is pure and unit-tested in `frontend/src/lib/person/genealogy/panZoom.ts`:
+  `clampZoom`, `parsePanZoomParams`/`serializePanZoomParams`, `screenDeltaToSvg`,
+  `zoomAtPoint` (centroid-anchored), `clampPan` (edge-clamp), `recentreOn`, `lerpView`.
+- Pan offsets shift the viewBox centre; zoom scales its width/height. The default
+  `{x:0, y:0, z:1}` already frames the whole tree, so **fit-to-screen is a reset to the
+  default** — no bounding-box recomputation.
+- DOM event wiring lives in the `panZoomGestures` action (pointer/wheel/pinch + inertia,
+  reduced-motion aware) and a keyboard handler on the SVG; both delegate to the pure module.
+
+## Consequences
+
+- **NFR-MAINT-001 (library pinning + feature-flag fallback) is moot** — no library is
+  adopted. The "swap-out point" is `panZoom.ts` + `panZoomGestures.ts`.
+- Text stays vector-crisp at any zoom (SVG-native scaling), satisfying US-PAN-002 AC5.
+- The #689 gutter and the #361 seeded-rank invariant are untouched by the pan/zoom layer.
+- Geometry is testable in the fast node project; only the DOM glue needs the browser project.
+- Trade-off: we own the inertia/pinch code (~a few hundred lines across the action) rather
+  than delegating it. This is acceptable given the testability and zero-dependency wins.
+
+The issue body's OQ-007 row is updated to point at this ADR.