Align slug transliteration with bDS2

specs/bds.allium

@@ -74,9 +74,9 @@ use "./metadata_diff.allium" as metadata_diff   -- DB/filesystem diff and rebuil
 
 -- Resolved questions:
 --
--- 1. Slug generation scope: only German and English letters are used.
---    Verify transliteration preserves the established bDS behaviour for
---    ä/ö/ü/ß/ÄÖÜ.
+-- 1. Slug generation is identical to bDS2: replace ß with "ss", normalize to
+--    Unicode NFD, discard non-ASCII code points, lowercase, hyphenate
+--    non-alphanumeric runs, and trim hyphens. Thus ä/ö/ü become a/o/u.
 --
 -- 2. Liquid subset: see template.allium for the exact subset.
 --    Only 5 tags, 4 standard filters, 2 custom filters, 5 operators.

specs/post.allium

@@ -14,10 +14,11 @@ enum PostStatus {
 value Slug {
     value: String
 
-    -- Generated by: transliterate unicode to ASCII, lowercase,
-    -- replace [^a-z0-9]+ with hyphens, strip leading/trailing hyphens
-    -- Transliteration scope: only German (ä/ö/ü/ß/ÄÖÜ) and English letters used.
-    -- Verify transliteration matches the established bDS behaviour for this set.
+    -- Generated exactly as bDS2: replace ß with "ss", normalize to Unicode NFD,
+    -- discard every non-ASCII code point, lowercase, replace [^a-z0-9]+ with
+    -- hyphens, then strip leading/trailing hyphens.
+    -- Therefore ä/ö/ü/Ä/Ö/Ü become a/o/u/A/O/U through NFD decomposition;
+    -- examples: "Über" -> "uber", "ÄÖÜäöüß" -> "aouaouss".
     -- Uniqueness: tries base, then {slug}-2, {slug}-3, … (unbounded numeric suffix)
 }