feat(vm-core): restore move-only Continuation — eliminate Arc/share_handle (K1, SPEC-VM-021)

[proboscis]

Summary

Restores the move-only Continuation law (SPEC-VM-021) that was eroded by PR #404's Arc<Mutex<Option<>>> + share_handle() mechanism. Preserves #404's exception-propagation semantics via a lighter-weight Py<PyK> handle approach.

Core changes (K1 coupling core — B2 ⇔ B3 ⇔ B6 ⇔ B10 ⇔ B12):

• continuation.rs: Continuation.chain is now plain Option<DetachedFiberChain> — no Arc, no Mutex. share_handle() removed entirely. take() is self.chain.take() (Option::take, not lock+take).
• value.rs: Added is_generator_handler() trait method to Callable — differentiates Python generator handlers (need PyK backup for exception recovery across yields) from synchronous Rust handlers (consume continuation directly via Value::Continuation).
• step.rs: eval_perform / eval_perform_with_k branch on is_generator_handler():
  • Generator path (Python @do): wraps k in PyK, stashes Py<PyK> handle for exception recovery, passes Value::Opaque(PyShared(PyK)) to call_handler
  • Synchronous path (Rust): passes Value::Continuation(k) directly, no backup needed
• vm.rs: pending_handler_chain_backup: Option<Continuation> → pending_handler_k_handle: Option<Py<PyK>>
• frame.rs: Frame::Program.chain_backup → handler_k_handle: Option<Py<PyK>>
• python_generator_stream.rs: PythonCallable::is_generator_handler() returns true
• invariants.rs: Rewritten to scan Py<PyK> handles instead of Arc cells

Bug fix: Stale-backup-leak — when a generator handler returns non-Expand DoCtrl, the backup handle now drops instead of persisting to corrupt the next Program frame.

Documentation: Decision record D19, constraint graph B3 row updated.

Acceptance Criteria Evidence

1. Move-only Continuation restored

• continuation.rs: chain: Option<DetachedFiberChain> (line ~252), no Arc/Mutex anywhere
• share_handle() removed — grep confirms zero occurrences
• take() uses self.chain.take() (line ~277)

2. PR #404 exception-propagation preserved

All 6 regression tests pass:

test_handler_bind_exception_propagation.py (6 tests) — PASSED

3. Guard layer tests resurrected (6 tests)

test_move_semantics_architecture.py:
  test_chain_field_is_plain_option — PASSED
  test_no_shared_handle_api — PASSED
  test_one_shot_via_option_take — PASSED
  test_vm_does_not_store_continuation — PASSED
  test_frame_does_not_store_continuation — PASSED
  test_value_continuation_panics_on_clone — PASSED

4. Stale-backup-leak regression test

vm_tests.rs: test_stale_backup_does_not_leak_for_non_expand_generator_handler — PASSED

5. Full test suites

• Rust (default features): 28 tests passed
• Rust (invariant-checks): 36 tests passed (8 invariant tests)
• Python integration: 541 passed, 1 pre-existing failure (unrelated doc test on main)
• Python guard layer: 6 passed

6. Decision record & constraint graph

• D19 added to docs/crystallization/decision-records.md
• B3 row updated in docs/crystallization/constraint-graph.md

Files Changed (11 files, +549 −335)

File	Change
continuation.rs	Remove Arc/Mutex/share_handle, plain Option chain
value.rs	Add is_generator_handler() trait method
step.rs	Branch eval_perform on handler type; fix stale-backup leak
vm.rs	pending_handler_k_handle: Option<Py<PyK>>
frame.rs	handler_k_handle: Option<Py<PyK>>
python_generator_stream.rs	Override is_generator_handler() → true
invariants.rs	Rewrite for Py handle scanning
vm_tests.rs	Add stale-backup-leak regression test
test_move_semantics_architecture.py	Rewrite 6 guard layer tests
decision-records.md	D19: K1 restoration
constraint-graph.md	B3 row evidence update

Reference

Issue: vm-k1-restore-move-only-continuation

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