Add optional Oracle 26ai Deep Data Security variant

Reimplements the same 4-user source-access matrix using Oracle AI
Database 26ai's Deep Data Security (DDS) — VPD's declarative SQL
successor. Coexists with the VPD demo (ddsuser_* / dds_* prefixes,
MAC intentionally not enabled on shared views).

- sql/adb/13_dds_variant.sql: CREATE END USER + CREATE DATA ROLE +
  CREATE DATA GRANT for the same 4-user matrix; row-filter and
  column-mask variants shown as commented examples.
- docs/05-dds-variant.md: prereqs (23.26.2+, COMPATIBLE>=20.0),
  VPD <-> DDS 1:1 mapping table, run + teardown snippets.
- .env.example: DDSUSER_*_PASSWORD block (3b).
- README.md: tree + "더 깊이" link.

Not wired into run.sh — kept manual since DDS requires 26ai.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.env.example

@@ -32,6 +32,14 @@ export VPDUSER_PG_PASSWORD="RowFilter#Pg2026"
 export VPDUSER_BOTH_PASSWORD="RowFilter#Both26"
 export VPDUSER_NONE_PASSWORD="RowFilter#None26"
 
+# --- (3b) OPTIONAL — DDS 변형 데모용 엔드유저 비밀번호 ---
+# sql/adb/13_dds_variant.sql (Oracle 26ai Deep Data Security 재구현) 에서만 사용.
+# 동일한 4-user 매트릭스를 declarative Data Grants 로 재현. VPD 데모와 공존.
+export DDSUSER_MY_PASSWORD="DdsGrant#My2026"
+export DDSUSER_PG_PASSWORD="DdsGrant#Pg2026"
+export DDSUSER_BOTH_PASSWORD="DdsGrant#Both26"
+export DDSUSER_NONE_PASSWORD="DdsGrant#None26"
+
 # --- (4) 원격 Postgres (AWS RDS, Cloud SQL, ...) ---
 # sql/source/postgres_setup.sql 가 여기로 customers 테이블/seed 생성.
 # ADB 의 RDS_POSTGRES_LINK 가 이 인스턴스를 가리킴.