record ADR-0035: a slug belongs to the bundle, not to a variant

Confirms ADR-0009's "same path" clause against the alternative that was on the
table: a Bengali variant at its own Bengali address. Chosen: one path per bundle,
prefixed per language, so identity stays stable across translations (invariant 3)
and the resolver keeps one lookup direction.

The declined option is named in the ADR so it is not re-proposed as new. Cost is
stated too — a Bengali reader sees a Latin address, and reversing this later needs
an alias for every published path.

No code moves: `slug` is still unread. Recorded now because nothing is published
yet, which is the only cheap moment a permalink decision has.

docs/decisions.md

@@ -422,3 +422,21 @@ that grows with the theme; a site root cannot yet add or override a key, so a th
 must write them in its own block.
 Revisit if: a site root needs its own chrome strings. That is the settings cascade's problem
 (`ideas/deferred-decisions.md`), not a second table.
+
+## ADR-0035 — A slug belongs to the bundle, not to a variant
+Date: 2026-07-30 · Status: accepted (confirms ADR-0009's "same path" clause against a per-language address)
+Decision: `slug` renames the bundle in every language, so a variant is always reached at the bundle's one
+path under its language prefix — `/posts/hello-world/` and `/bn/posts/hello-world/`, never a Bengali
+spelling of the address. Where two variants declare different slugs the declaration is ambiguous: log both
+and keep the derived path, the same way colliding keys and contested aliases are handled (ADR-0029). The
+engine never derives a slug from a title in any language; a path changes only because an author wrote one.
+Why: the alternative makes the path a function of (key, language) rather than of the key alone, which
+costs a second lookup direction in the resolver and a permalink shape that differs per language — while
+invariant 3 says identity is stable across translations and ADR-0009 already promised the same path under
+a prefix. A Bengali-looking address is a real reader benefit, but not one worth a second identity rule
+before anything is published.
+Consequence: cheap — one path shape, one lookup direction, `/bn/` stays pure prefixing, and a translation
+can be added or dropped without touching a URL. Expensive — a Bengali reader sees a Latin address, and
+changing that later needs this decision reversed plus an alias for every path already published.
+Revisit if: Bengali becomes the dominant language of the site — the trigger ADR-0009 already names. That
+is a default-language change, not per-variant slugs.