docs(stammbaum): document gutter + persons.generation column (#689)

db-orm.puml: persons gains a generation : SMALLINT attribute mirroring
the V70 column. No FK change, so db-relationships.puml is unaffected.

stammbaum-tree-spec.html:
- impl-ref table: replace "Gen label" with "Gutter label" + new
  "Gutter stripe underlay" rows describing the role="text" wrapper,
  un-shifted source-truth value, and below-md hidden state.
- light + dark colour-table rows updated to "Gutter label" /
  "Gutter stripe" with the new var(--c-ink-2) / var(--c-gutter-stripe)
  swatches.
- "Generationen ▾" filter chip mocks removed from desktop and tablet
  layout sections (the filter UI was de-scoped from this PR).

Inline visual mockup SVGs that still show pre-gutter labelling are
out of scope per the issue body — the impl-ref table is the
authoritative source for this PR.

Refs #689

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

docs/architecture/db/db-orm.puml

@@ -186,6 +186,7 @@ package "Persons" {
     notes : TEXT
     birth_year : INTEGER
     death_year : INTEGER
+    generation : SMALLINT
     family_member : BOOLEAN NOT NULL
     source_ref : VARCHAR(255) UNIQUE
     provisional : BOOLEAN NOT NULL