Add sealed module: modelling domains with sealed types and exhaustive switch

Payment model, permits and the three modifiers, default-branch trap, generic and recursive hierarchies, visitor comparison, run-time enforcement, modules. Transcripts from JDK 21, 25 and 27.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01TF9JWFvJSNm6HVzswzZU5a

README.md

@@ -66,6 +66,7 @@ These modules keep only sources, a `run.sh` and their transcripts (`output/`). T
 | [`jep513/`](jep513) | Flexible Constructor Bodies in Java 25 (JEP 513): Validate Before super() | statements before `super(...)`/`this(...)`, the parent-calls-child bug and its fix, twelve compile errors that remain with the 25 and 27 wordings side by side, `--release` rejection |
 | [`jep511/`](jep511) | Module Import Declarations in Java 25 (JEP 511) | `import module java.base;` against seven single imports, ambiguity errors and how single-type, on-demand and same-package names beat a module import, what one module import does and does not bring in (`java.sql` does not give `java.base`; `java.se` needs `--add-modules`), named modules, JEP 512 compact files, jshell's default imports, identical bytecode |
 | [`patterns/`](patterns) | Pattern Matching in Java: switch, Record Patterns and Primitive Patterns (JDK 21 to 27) | type and record patterns, guards and dominance errors, sealed exhaustiveness (with JDK 27's new `missing patterns` lines), `MatchException` from a throwing accessor and from separate compilation, unnamed patterns, primitive patterns with `--enable-preview` and the preview class-file trap |
+| [`sealed/`](sealed) | Sealed Classes and Interfaces: Modelling Domains with Exhaustive switch | a `Payment` model, `permits` and the three subclass modifiers, the `default` trap, generic and recursive hierarchies, the visitor for comparison, run-time enforcement and modules |
 
 ## Captured output (`docs/output/`)