GROOVY-12227: GeneratedDispatcher: avoid runtime class definition so packed closures work in native images

[paulk-asert]

Two layered changes to the GROOVY-12151 packed-closure machinery (GEP-27):

1. ClosureWriter now emits into each packed hosting class a private
   static $packedDispatchersFactory$ whose body adapts the class's three
   private static dispatch tables to their functional interfaces through
   ordinary bytecode-level LambdaMetafactory invokedynamic sites and
   returns them as one Bundle. The dispatcher accessor's bootstrap becomes
   the new four-argument IndyInterface.packedDispatchers, which receives
   that factory as a CONSTANT_MethodHandle bootstrap argument.

2. GeneratedDispatcher.bootstrap (four-argument form) just invokes the
   factory into a ConstantCallSite: linking needs neither a runtime
   Lookup.findStatic nor a programmatic LambdaMetafactory call — an
   undeclared reflective lookup and a run-time class definition, the two
   operations ahead-of-time runtimes restrict, so the linkage suits any
   such environment. Under GraalVM native image, the verified case, the
   factory's sites are pre-processed at image build time and its method
   references reach the class's own private tables without reflection
   metadata.

On a regular JVM the linkage is equivalent: the VM spins the same three
hidden classes when it links the factory's sites, with the cost moving
from bootstrap-time programmatic LMF to first-invocation indy linkage
(per packed class: one extra synthetic method, three indy sites, one
bootstrap argument; the accessor shape is unchanged). The Bundle
constructor is publicized because the compiler-emitted factory in the
hosting class's own package constructs it directly.

The previous three-argument bootstrap is retained verbatim for class
files emitted by earlier 6.0 pre-releases, which keep linking through
findStatic plus programmatic LambdaMetafactory. Conversely, class files
in the new format need this runtime to link; both directions only
concern unreleased 6.0 snapshots.

Verified on GraalVM 25.2.4 (native-image 25.0.4): the packed repro that
previously failed with 'Classes cannot be defined at runtime ...
M$$Lambda...' now runs correctly (single emitted class, 30MB image,
~12ms total run time), and the tracing agent records zero
packedDispatch entries for the new bytecode.

PackedDispatcherFactoryTest covers every dispatch shape and the
propagation of undeclared checked exceptions through both class-file
formats -- the legacy format recreated by downgrading a freshly
compiled accessor's bootstrap reference from the four-argument to the
three-argument form, the only difference between the two formats. The
test serializes its system-property mutation against other
property-touching tests with @ResourceLock(SYSTEM_PROPERTIES).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.0025%. Comparing base (32d6a70) to head (6fcc0a8).
⚠️ Report is 2 commits behind head on master.

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2765        +/-   ##
==================================================
+ Coverage     69.9972%   70.0025%   +0.0053%     
- Complexity      35557      35560         +3     
==================================================
  Files            1558       1558                
  Lines          131731     131751        +20     
  Branches        24178      24178                
==================================================
+ Hits            92208      92229        +21     
  Misses          31176      31176                
+ Partials         8347       8346         -1     

Files with missing lines	Coverage Δ
...rg/codehaus/groovy/classgen/asm/ClosureWriter.java	90.0404% <100.0000%> (+0.2473%)	⬆️
...g/codehaus/groovy/runtime/GeneratedDispatcher.java	87.5000% <100.0000%> (+0.5435%)	⬆️
...org/codehaus/groovy/vmplugin/v8/IndyInterface.java	84.0659% <100.0000%> (+0.0880%)	⬆️

... and 7 files with indirect coverage changes

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