Document that source keys are case-sensitive

Keys differing only in case denote different Sources: dbnames on Linux
MySQL/MariaDB (lower_case_table_names=0) can differ only in case, and
the graph's wiki_id stores the wiki ID verbatim, so case-folding is not
an option.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: malberts

docs/reference/subject-format.md

@@ -182,7 +182,8 @@ A Subject id has two textual forms ([ADR 23](../adr/023-subject-sources.md)):
 - The **source-qualified form** `source:localId`, referencing a Subject from a specific Source. The part
   before the first `:` is the source key (`[A-Za-z0-9+_-]+`; for wikis, the
   [MediaWiki Wiki ID](https://www.mediawiki.org/wiki/Manual:Wiki_ID)); the rest is the localId, whose
-  grammar is owned by its Source. Until Sources can define their own grammars, localIds are restricted to
+  grammar is owned by its Source. Source keys are case-sensitive: keys that differ only in case denote
+  different Sources. Until Sources can define their own grammars, localIds are restricted to
   URL-path-safe characters (RFC 3986 pchar, excluding percent-encoding). An id that explicitly names the
   local source is equivalent to its bare form and is normalized to it.