Complete native async task integration

[kleeedolinux]

Summary

Complete coroutines, async functions, awaiting, cooperative cancellation, and
scoped async cleanup through one backend-neutral HIR/MIR contract.

This change:

• adds typed cold tasks, await, async closures, async defer, structured task
  groups, explicit cancellation sources/tokens, and cancellation masking;
• lowers coroutine state, suspension, cleanup, cancellation, panic, and task
  ownership into verified HIR and canonical MIR;
• executes the same contract through the MIR interpreter and LLVM;
• connects compiler-created task frames to the native scheduler;
• publishes exact ready and suspended frame roots through the scheduler/collector
  lifecycle and restores relocated roots before polling;
• adds native ABI 1.12 task-frame and cancellation operations without claiming
  ABI 2 production-GC support;
• marks the corresponding async and scheduler/GC roadmap work complete.

Architecture traceability

• Authorizing architecture section or ADR:
  • ADR 0068: Typed Async Tasks, Actors, and Distribution
  • ADR 0077: Scheduler Mutator and Task Root Binding
  • ADR 0078: Native ABI 2 Writable-Root Coexistence
  • ADR 0079: Native Task-Frame and Cancellation ABI
  • architecture/05-runtime-and-abi.md
• New or changed public contract:
  • Task.cancellationSource(), Task.cancelToken(source), and
    Task.cancel(source)
  • typed Task.group and Task.start
  • cold Task<T> creation, prefix await, async functions/closures, and
    suspension-capable async defer
  • backend-neutral Synchronizes and Suspends effects
  • native ABI 1.12 compiler-frame, task, group, completion, and cancellation
    entries
• Architecture documents, examples, or terminology updated:
  • added accepted ADR 0079;
  • synchronized the runtime/ABI architecture, ADR 0078, closed design
    decisions, foundational Task metadata, and API baseline;
  • completed both affected ROADMAP.md entries.

Verification

• Tests were added or updated before implementation where behavior changed.
• Positive behavior is covered.
• Negative/rejection boundaries are covered.
• Convention, consistency, and regression coverage is present where relevant.
• Cross-backend or differential coverage is present where relevant.
• cargo fmt --all -- --check
• cargo check --workspace --all-targets
• cargo test --workspace --all-targets
• cargo clippy --workspace --all-targets -- -D warnings

If a check was not run, explain why:

cargo test --workspace passed completely. The all-target command was also
started and passed the compiler, runtime, architecture, interpreter, LLVM, C,
and other test suites reached before it entered the long harness-free compiler
and scheduler benchmarks. It was manually interrupted while those benchmarks
were still succeeding, so the complete all-target command did not return a
final status.

Additional focused suites passed, including the full MIR, LLVM, native runtime,
native ABI, standard-library baseline, and architecture test suites.

Review notes

• No dynamic typing, runtime string lookup, broad reflection, or universal-table behavior was introduced.
• HIR/MIR remain backend-neutral.
• No generated artifacts, dependency caches, credentials, or editor files are included.
• This is ready for technical review.

Native ABI 1.12 deliberately retains stable managed-reference tokens. It does
not advertise ABI 2 or satisfy the production moving-GC profile. Production GC
integration remains gated on the complete ABI 2 writable-root composition.

The experimental C backend rejects coroutine runtime requirements before
emission and has no synchronous or dynamic fallback.