fix(jcpan): unblock ActiveResource dependency chain (Encode tags, SAX undef parity)

[fglock]

Summary

Investigates and fixes two of the three failures uncovered by jcpan -t ActiveResource.

ActiveResource itself never reaches its own test suite — it fails because of a
chain of dependencies, each broken for a different reason:

ActiveResource
├── Class::Accessor::Lvalue   (XS dep "Want" — no Java port; deferred)
└── XML::Hash
    └── Test::XML
        └── XML::SemanticDiff (2 fails fixed here)

See dev/modules/active_resource.md (added in
this PR) for the full breakdown.

What this PR does

1. dev/modules/active_resource.md — full diagnosis of all three blockers.
2. dev/modules/want.md — detailed port plan for the deferred Want
   blocker (see "Out of scope" below).
3. Fix return inside a literal can break JVM stack #1: Encode %EXPORT_TAGS — was missing :all and :default, so
   use Encode qw(:all) died with "all" is not defined in %Encode::EXPORT_TAGS.
   Adds a reusable defineDefaultAndAllTags() helper in PerlModuleBase and
   wires it up in Encode.java. Now matches core perl exactly:
   keys %Encode::EXPORT_TAGS = (all, default, fallback_all, fallbacks).
4. Fix -i switch wip #2: XML::Parser::Expat current_element push/pop timing — real
   libexpat updates Context AFTER the user Start handler returns and BEFORE
   the user End handler runs. PerlOnJava had the opposite timing, so
   current_element() returned the wrong element from inside Start/End. This
   broke XML::SemanticDiff (its Text accumulator misattributed text to the
   just-started child, turning empty-element CData from undef into '').
   Fixed by reordering the Context push/pop in XMLParserExpat.java.

Out of scope (follow-up)

• Implementing the Want XS module (large, separate PR). Without it,
  Class::Accessor::Lvalue and therefore ActiveResource itself remain
  unreachable. Detailed port plan in
  dev/modules/want.md — recommends starting with
  Option A1: a runtime lvalue-context flag in PerlOnJava plus a Pure-Perl
  Want shim covering LVALUE/RVALUE/ASSIGN/LIST/SCALAR/VOID and
  rreturn/lnoreturn. That subset is enough to unblock
  Class::Accessor::Lvalue and therefore ActiveResource; the more exotic
  OBJECT('Pkg') / CHAIN(N) introspection is left for later.

Test plan

• make passes locally (all unit tests).
• New regression tests:
  • src/test/resources/unit/encode_export_tags.t — 8 subtests
  • src/test/resources/unit/xml_parser_current_element.t — 12 subtests
• Bundled XML::Parser suite (src/test/resources/module/XML-Parser/t/*.t):
  45 files / 434 tests, all pass (no regression from the Context timing change).
• XML::SemanticDiff standalone: 18 files / 47 tests, all pass; the 2
  previously-failing subtests in t/16zero_to_empty_str_cmp.t now pass.
• Encode parity: keys %Encode::EXPORT_TAGS now matches system perl.

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