Fix Pod::Spell by loading Encode for encoding layers

[fglock]

Summary

• load Encode.pm when applying a PerlIO :encoding(...) layer
• match the visible side effect provided by standard Perl's PerlIO::encoding
• add a regression test covering Encode::encode availability

Root cause

Pod::Wordlist opens its bundled wordlist with :encoding(UTF-8).
Standard Perl loads Encode while applying that layer, and Pod::Spell
later calls Encode::encode. PerlOnJava performed the charset conversion
directly without loading Encode, so the call was undefined.

Test plan

• prove src/test/resources/unit/perlio_encoding_loads_encode.t
• ./jperl src/test/resources/unit/perlio_encoding_loads_encode.t
• minimal interpreter-backend parity check
• ./jcpan Pod::Spell — all 8 test files and 54 tests pass; install succeeds
• make — compilation and 223 unit tests passed, then the unrelated existing
  unit/refcount/weaken_edge_cases.t test 37 failed; the same failure reproduces
  when that test is run standalone

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