docs(D-W6.5): function-store + cmop-bootstrap reproducers; drift narrowed

[fglock]

Summary

Two more focused reproducers + cumulative D-W6 findings.

What landed

src/test/resources/unit/refcount/drift/:

• function_hash_store.t (18 tests) — five patterns of the
  sub setit { $METAS{$_[0]} = $_[1] } shape that
  Class::MOP::store_metaclass_by_name uses.
• cmop_bootstrap.t (14 tests) — exact MOP shape: our %METAS
  global + weakened-back-ref Attribute + MetaClass with DESTROY.

Both pass on master AND with the walker gate disabled.

Cumulative D-W6 findings (sessions 1–5)

PerlOnJava's cooperative refcount handles ALL of these correctly
with the gate disabled. None is the drift source.

Reproducer	Tests	Patterns covered
sub_install.t	12	Glob assign, named sub, loop install, temp drop, nested install
closure_capture.t	8	Single → 5-layer wrap, 20-closure chain
hash_slot.t	14	Direct slot, package global, 50-entry registry, overwrite
weak_metaclass.t	14	Weak slot + outer keepalive, 20-loop, rescue
function_hash_store.t	18	Function-internal store via $_[N] and shift
cmop_bootstrap.t	14	%METAS + weak attr back-ref + DESTROY
Total	80

What this means

The actual Class::MOP drift is something more specific than any
of these clean shapes. Likely candidates left:

1. Multi-level metaclass relationships —
   Class::MOP::Class has 7+ ancestors via use parent.
2. Attribute->attach_to_class called from many scopes.
3. Method modifiers (add_around_method_modifier etc.).
4. use parent / @ISA compile-time setup with mixin classes.

Suggestion for next session (in design doc)

Stop writing reproducers from scratch. Bisect the real
Class::MOP source: take cmop_bootstrap.t and gradually add
features from Class::MOP::Class.pm until something breaks
without the gate.

The first feature to flip the test red is the smoking gun.

Test plan

• make (build + unit tests) green.
• All four new reproducers pass on master (gate active).
• All four pass with the gate disabled in a probe build.

Generated with Devin

Co-Authored-By: Devin <158243242+devin-ai-integration[bot]@users.noreply.github.com>