Expand Object::Pad compatibility with :accessor fields

[fglock]

Summary

Expand PerlOnJava's Object::Pad compatibility beyond the native compiler's
core class, field, and method syntax. In particular, support the
Object::Pad field attribute :accessor.

PerlOnJava currently advertises Object::Pad 0.66, so optional CPAN tests
guarded by Test::Requires 'Object::Pad' run. However, both backends reject
Object::Pad-specific MOP syntax such as :accessor at compile time.

Reproducer

use Object::Pad;

class Example {
    has $value : accessor = 'value';
}

Standard Object::Pad accepts this declaration. PerlOnJava reports a syntax
error near : accessor = on both the JVM and interpreter backends.

CPAN evidence

MooseX::LocalAttribute 0.05 regressed in CPAN tester run
20260827-171022-1876. All of its non-Object::Pad tests pass. Its optional
t/objectpad.t runs because Object::Pad is present, then cannot compile
Tester::ObjectPad, which declares two fields using :accessor.

The current feature matrix already documents that Object::Pad MOP extensions
are not included; this issue tracks closing that compatibility gap rather than
an undocumented semantic regression.

Ecosystem impact

MetaCPAN reports 135 unique reverse-dependant distributions for Object::Pad,
including 31 updated in the preceding 12 months. Recent dependants include
OpenTelemetry, DBIx-Fast, Device-Chip, Sys-Async-Virt, and
Object-PadX-Enum. Those counts cover all Object::Pad APIs, not only
:accessor, but demonstrate material ecosystem relevance.

Suggested scope and acceptance coverage

• Parse and implement has $field :accessor = EXPR with Object::Pad-compatible
  generated accessors and initialization behavior.
• Define the supported Object::Pad extension surface explicitly, so a module
  is not presented as fully compatible merely because core class syntax works.
• Add focused project-owned tests for :accessor fields and generated
  read/write accessors on both backends.
• Use MooseX::LocalAttribute t/objectpad.t as downstream regression
  coverage.