Make snapshot attribute validation enforce a real rule

Closes gap G06. checkNoUnknownAttributes was named for a rule it did not
implement: it rejected blank keys, which is not an allow-list, so a snapshot
carrying malformed or shadowing attribute names passed a check whose name said
otherwise. A rule that passes for the wrong reason is worse than an absent one,
because the report is read as evidence.

raw_attributes_well_formed requires each key to be a valid LDAP attribute
descriptor, forbids shadowing an attribute the canonical model owns (uid, cn,
mail) in any casing, and rejects keys differing only by case, which LDAP treats
as one attribute. spec/ldap-schema.yaml carries the same wording.

Two corrections to the gap's description rather than implementations of it. An
allow-list is not derivable: ldapAttributes is defined as what the canonical
model does not cover, so the schema cannot enumerate what may appear there. And
the reference constraint already existed -- checkValidGroupMemberships only
checks emptiness, but the semantic rule checkReferencedUsersExist resolves every
member against the user set.

Neutering the rule fails four of the five new tests.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NV9oijZukGyGbRQGGKnK4P

Assistant: claude-code
Assistant-Model: opus
Assistant-Process: 713576@bnt-lap001
Assistant-Session: 384c511d-9bce-4cb8-a676-2aef6c0c8df6

spec/ldap-schema.yaml

@@ -76,8 +76,13 @@ validation_rules:
       description: "All DNs must conform to the base_dn and OU layout above."
     - name: required_attributes_present
       description: "Every entry must carry all required attributes for its OU."
-    - name: no_unknown_attributes
-      description: "No attributes outside the allowed set may appear."
+    - name: raw_attributes_well_formed
+      description: >
+        Raw ldapAttributes keys must be valid LDAP attribute descriptors, must
+        not shadow an attribute the canonical model owns (uid, cn, mail), and
+        must not repeat one attribute under different casing. This is not an
+        allow-list: ldapAttributes carries what the canonical model does not
+        name, so no permitted-name set can be derived from the schema.
     - name: valid_group_memberships
       description: "All member values must be non-empty valid DNs."
   semantic: