SPEC-VM-021 Step 7: Fix 5 architecture violations for single-owner continuations

[proboscis]

Summary

• Violation 1: return_to_continuation() in dispatch.rs replaced with return_to_fiber_id() — returns FiberId instead of manufacturing Continuation from stored frame data. Callers now use capture_live_continuation() (the approved live-topology method).
• Violation 2+3: ContId type deleted entirely. derived_cont_id() removed. Replaced with Continuation::identity() -> Option<FiberId> and fresh_pending_cont_id() -> u64 for pending continuations.
• Violation 4: DoCtrl::EvalInScope { scope: Continuation } changed to { scope_fiber: FiberId } — no Continuation needed for scope reference.
• Violation 5: Scheduler park-and-continue pattern fixed — FiberId extracted before parking k_user, Continuation created after k_user is consumed. Ensures single owner at all times.

Test results

• Rust vm-core: 55/55 pass (53 + 2 integration), 1 ignored
• Rust core-effects: 82/84 pass, 2 pre-existing env failures (missing doeff Python module)
• Python: 79/97 pass, 18 pre-existing failures on main
• Zero regressions

Files changed (12)

• doeff-vm-core: continuation.rs, do_ctrl.rs, ids.rs, lib.rs, vm/dispatch.rs, vm/step.rs, vm/vm_trace.rs, vm_tests.rs
• doeff-core-effects: scheduler/mod.rs, handlers/mod.rs
• doeff-vm: lib.rs, pyvm.rs

Test plan

• cargo test for doeff-vm-core (55 pass)
• cargo test for doeff-core-effects (82 pass, 2 pre-existing env failures)
• cargo build for doeff-vm (compiles clean)
• Python integration tests (79 pass, 18 pre-existing failures)
• Verify zero regressions vs main branch

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