fix(stammbaum): address re-review nits — opaque rail, stale docs, rail clarity (#692)

- Rail chip background opaque (was /85) so G{n} labels stay AA-legible over
  tree content (Leonie).
- Rail effect: replace the reactKey hack with an inputsFinite guard that both
  tracks deps and guards NaN; name the fallback-stack magics; correct the stale
  'xMidYMid' comment (the CTM mapping is preserveAspectRatio-agnostic) (Felix/Markus).
- GLOSSARY zoom range 0.25–3.0 → 0.25–10; ADR-027 preserveAspectRatio note
  xMidYMid → xMinYMin (Elicit traceability).

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

docs/adr/027-stammbaum-custom-viewbox-pan-zoom.md

@@ -16,7 +16,8 @@ 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
+layout bounding box, `preserveAspectRatio="xMinYMin meet"` so a fresh visit anchors to the
+tree's top-left corner) — not by applying a CSS
 `transform`. The `panzoom` library operates by writing `transform` to a DOM node. Adopting
 it would mean: