Add inheritance module: SINGLE_TABLE vs JOINED vs TABLE_PER_CLASS on Hibernate 7.4.13.Final

Same payment hierarchy mapped three ways plus @MappedSuperclass, run with plain Hibernate on H2.
Tests assert and write the generated DDL, the INSERT/SELECT SQL, row/insert/FK/nullability counts and
the three failures (NOT NULL on a SINGLE_TABLE subclass column, IDENTITY with TABLE_PER_CLASS, querying a
@MappedSuperclass); Bench writes indicative polymorphic-query timings. No docs folder: depth lives in the post.

inheritance/output/04-polymorphic-timings.txt

@@ -0,0 +1,20 @@
+INDICATIVE ONLY - timings drift between runs and machines; nothing here is asserted.
+Hibernate 7.4.13.Final, H2 in memory, JDK 25.0.5+8-LTS, 2 vCPU
+150000 payments (50000 card, 50000 bank transfer, 50000 wallet), read through a StatelessSession, median of 15 runs after 5 warm-ups
+
+strategy         query                                            rows  median ms
+single-table     all payments (polymorphic)                     150000      247.2
+single-table     one subclass (CardPayment)                      50000       82.4
+single-table     count where amount > 500 (root col)                 1       18.4
+single-table     first 50 by id (polymorphic)                       50        0.3
+
+joined           all payments (polymorphic)                     150000      401.5
+joined           one subclass (CardPayment)                      50000      107.1
+joined           count where amount > 500 (root col)                 1       21.6
+joined           first 50 by id (polymorphic)                       50        0.3
+
+table-per-class  all payments (polymorphic)                     150000      422.0
+table-per-class  one subclass (CardPayment)                      50000       82.1
+table-per-class  count where amount > 500 (root col)                 1       56.0
+table-per-class  first 50 by id (polymorphic)                       50      165.8
+