Modernize Generate.java to Java 25 with modern language features

Rewritten using Java 9-25 features:
  - Compact source file with void main() (JEP 512)
  - Module import: import module java.base (JEP 511)
  - Record for Snippet data access (JEP 395)
  - Sealed interface + record patterns for NavArrow (JEP 409/441)
  - Pattern matching for switch with null case (JEP 441)
  - Unnamed catch variable _ (JEP 456)
  - Stream.toList() (JDK 16)
  - String.formatted() (JDK 15)
  - SequencedMap (JDK 21)
  - IO.println() (JDK 25)

Produces identical HTML output to generate.py.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges