chat: bump simplexmq pin + document cross-table simplex_name discriminator

Pin bump 5ee014dd -> c9c2d19 picks up the 8 simplexmq commits since the
last bump (parseBare lowercase fix, forwarded-param cleanup, ServerTests
+ agent end-to-end tests, TldRegistries removal, SNRC ABI decoder,
NameRecord/NameOwner module extraction).

Adds a brief comment on clearConflictingContactProfileSimplexName_
explaining why the audit's flagged cross-table collision (between
contact_profiles.simplex_name and group_profiles.simplex_name) is
structurally impossible: SimplexNameInfo's strEncode prefixes contact
names with '@' and group names with '#', so the stored bytes never
overlap between the two tables.

Query-plan regen deferred (the test is non-deterministic in CI / dev
sandbox — see prior 6c990696c).

cabal.project

@@ -21,7 +21,7 @@ constraints: zip +disable-bzip2 +disable-zstd
 source-repository-package
     type: git
     location: https://github.com/simplex-chat/simplexmq.git
-    tag: 5ee014ddccd11e612fe873a7150ccf78b81a0ad2
+    tag: c9c2d19074a809ba505f393b41aa20ac7b437aa7
 
 source-repository-package
     type: git