Lower async functions to backend-neutral resumable state machines

Parent: #439
Depends on: #440 and #442.

## Summary

Lower typed async functions into a backend-neutral resumable state-machine representation before LLVM emission.

## Required contract

- Split control flow at await sites and assign deterministic suspension states.
- Move locals that survive suspension into an explicit frame with target-correct size and alignment.
- Preserve return, loop, match, branch, and early-completion behavior across suspension.
- Represent ready, pending, failure, and cancellation according to the agreed semantic contract.
- Keep raw pointer values unchanged while documenting that pointee validity remains the program's responsibility.
- Support nested awaits, multiple awaits, generic specializations, and async calls returning variants or structs.
- Do not use LLVM coroutine intrinsics as the public or frontend contract; LLVM-specific optimization may be added behind the lowered representation.

## Completion criteria

- [ ] A backend-neutral async frame/state-machine IR is documented in code and consumed by LLVM.
- [ ] Generated LLVM verifies for x86_64, AArch64, and RV64 layouts.
- [ ] Locals live across exactly the required suspension states.
- [ ] Completed frames cannot resume again, and cancelled frames follow the defined cleanup contract.
- [ ] Regression tests cover branches, loops, match, nested calls, generics, and pointer-bearing frames.
- [ ] No valid async program reaches an unimplemented-node panic in LLVM.

This is a high-difficulty compiler task.

Start by reading the parent issue #439 and dependencies #440 and #442, then trace the existing async lowering path through LLVM emission. Define the backend-neutral frame and state-machine representation, and verify it against the listed completion criteria, including cross-target LLVM verification and regression coverage for control flow, generics, nested calls, and pointer-bearing frames.

Written by the indexing model from the issue text.