Remove obsolete native managed-invocation machinery - #133660

## Summary

Layer 3 of the stack for #123864, following #133658. The prior
func-eval-specific proposal is #126809.

This layer started as replacing the final native managed-call dispatch
with a managed UnmanagedCallersOnly entrypoint. It now moves most
debugger function-evaluation preparation, invocation, storage, and
copy-back into managed CoreLib while retaining the debugger-specific
control flow and GC-safe bootstrap in native code.

### Managed invocation and storage

- Add `Debugger.FunctionEvaluation`, a runtime-owned UCO entrypoint that
resolves the exact target method from its declaring `RuntimeType` and
rooted `IRuntimeMethodInfo`.
- Extend the shared reflection invocation emitter with debugger-specific
thunks that prepare arguments, invoke methods and constructors, copy
back writable arguments, and publish results.
- Emit `newobj` for debugger object construction instead of
preallocating an object and calling its constructor as an ordinary
method. Existing-object constructor calls remain supported, and
abstract/interface allocation is rejected.
- Keep the entire managed call-address vector registered through
`GCFrameRegistration`, using exact typed locals for memory-backed
by-value ref structs so their managed byrefs are reported by normal JIT
GC info.
- Handle object, primitive, enum, pointer, value-type, nullable,
literal, memory, register, and byref argument representations in managed
code while preserving native-endian storage and debugger copy-back
semantics.
- Align nullable behavior with the existing debugger contract: direct
value-type memory homes are mutated in place, while debugger handles are
not treated as writable nullable homes. Exact register-backed nullable
temporaries are boxed only after their MethodTable is instance-active.
- Move ordinary value-type result storage into temporary GC-described
external memory and defer boxing until after invocation and argument
copy-back. Keep byref-like results in persistent external storage and
raw byref results rooted directly in the debugger result slot.
- Create result handles in managed code only after the result is fully
prepared, and preserve object identity and updated field contents across
collections and later debugger inspection.

### Native debugger and GC integration

- Retain native method/signature validation, generic resolution, class
initialization, receiver validation inputs, register/home capture, abort
state, exception publication, completion, and result ownership.
- Capture contiguous register value-type snapshots before the first
GC-triggering managed transition and describe them with their exact type
layout.
- Root non-leaf register references and register value-type snapshots
with `ExternalMemoryHandle` registrations so collections during managed
preparation relocate their contents correctly.
- Represent external memory with a full tagged `TypeHandle`, allowing
the runtime and cDAC to distinguish inline value types, object-reference
slots, managed-byref slots, and unmanaged pointer/function-pointer
locals.
- Report each external-memory type's loader allocator during GC
scanning, keeping collectible layouts alive while external storage is
registered.
- Add dedicated GC-safe object-register copy-back and native-endian
scalar register copy-back paths.
- Remove stale native argument copies and architecture-specific packing
layers that are no longer needed, while preserving the
architecture-specific register-home logic required by debugger IPC.
- Treat the debugger UCO entrypoint as a runtime-internal reverse
transition for exception handling, and exclude its debugger-only
MethodDesc state on WebAssembly targets that do not build `debug/ee`.
- Update the cDAC ExternalMemoryHandles reader, descriptors, tests, and
authoritative contract documentation for the exact managed-local
representation.

### Correctness fixes included

- Avoid double relocation of overlapping byrefs in by-value ref structs
(#135245).
- Keep raw byref results rooted until deferred debugger completion
(#135247).
- Prevent stale non-leaf register values after a compacting GC
(#135249).
- Preserve current external-result fields across collections and later
inspection (#135251).
- Keep collectible loader allocators alive while external-memory layouts
are scanned (#135252).

## Validation

Validation used the existing internal MDbg/xUnit debugger-test harness
with a private CoreRun and ProjectK/CoreCLR-only configuration:

- Targeted nested-byref invariant under target-only `GCStress=0x3`:
**1/1 passed**.
- Revised normal matrix: **35/35 passed**.
- Targeted stress matrix: **4/4 passed**.
- Windows x64 and x86 Checked native, CoreLib, and ReadyToRun builds
completed with zero warnings and errors.
- cDAC unit tests and generated usage/documentation checks passed for
the ExternalMemoryHandle descriptor changes.

Coverage includes moving GC, overlapping and nested byrefs, memory and
register homes, non-leaf registers, receiver mutation, constructors,
primitive/native-endian copy-back, nullable memory homes,
byref/byref-like/value-type results, collectible generics and layouts,
nested and exception-time eval, safe/rude/exception-unwind aborts, and
post-eval inspection. Runtime hashes and loaded modules were audited;
each abort scenario completed the callback, successfully evaluated
`Echo(42)`, detached, and exited MDbg with code 100. Test configurations
were restored and no debugger/test processes survived the runs.

These results were recorded on the validated pre-refresh candidate. The
final stack rebase preserved the layer's net diff; builds and tests were
not repeated after the history rewrite at the user's request.

The final native invocation cleanup layer is #133660.

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Resolves #135245
Resolves #135247
Resolves #135249
Resolves #135251
Resolves #135252

> [!NOTE]
> This PR description and implementation were prepared with GitHub
Copilot. Commits that reuse am11's code retain the requested co-author
attribution.

---------

Co-authored-by: Adeel Mujahid <[email protected]>
Co-authored-by: Copilot App <[email protected]>