… group by IR level

Reorganize the multi-GPU e2e tests to match how the lowering stack is
layered. Each subdirectory hosts tests at one IR-abstraction level;
future phases (4-7) drop into their own subdir without touching anything
else.

Directory rename:
  test/gpu/symmetric_heap_dma/  →  test/gpu/multi_gpu/

The old name was misleading — most of the tests don't do DMA in the
conventional sense (the cacheline / allgather / rank variants use
vec-store + gpu.shuffle; the atomic variant uses atomicrmw; phases 5/6
will add real DMA later). The common thread is the symmetric-heap
fabric, not DMA.

New layout:
  test/gpu/multi_gpu/
    README.md                   # explains the layered structure
    handwritten/                # Phase 2 reference
      Makefile                  # self-contained; INPUT=cacheline|atomic|allgather
      cacheline.mlir            # was: air_sym_handwritten_cacheline.mlir
      atomic.mlir               # was: air_sym_handwritten_atomic.mlir
      allgather.mlir            # was: air_sym_handwritten_allgather.mlir
    air_rank/                   # Phase 3 (air.rank wrapping)
      Makefile                  # self-contained; INPUT=cacheline|allgather
      cacheline.mlir            # was: air_sym_with_rank_cacheline.mlir
      allgather.mlir            # was: air_sym_with_rank_allgather.mlir

Per-phase invocation: `make` instead of `bash run.sh`. Same default
behavior (NUM_RANKS=2, INPUT=cacheline). Make's dependency tracking
avoids re-running the lowering pipeline when only NUM_RANKS changes.

  make -C test/gpu/multi_gpu/handwritten                    # default
  make -C test/gpu/multi_gpu/handwritten INPUT=atomic
  make -C test/gpu/multi_gpu/handwritten INPUT=allgather
  make -C test/gpu/multi_gpu/handwritten NUM_RANKS=4
  make -C test/gpu/multi_gpu/air_rank                        # default
  make -C test/gpu/multi_gpu/air_rank INPUT=allgather
  make -C test/gpu/multi_gpu/handwritten clean

Why per-subdir self-contained Makefile (no _common.mk / no _common.sh):
  - Each phase's PR touches only its own subdir; no rebase conflicts on
    a shared file. Phases 2-7 had to re-resolve run.sh case-statement
    conflicts on every rebase under the old shared-script design (the
    cascade hit 4 conflicts during just the phase 2 rebase).
  - A shared include rots silently — one phase's edit can break another's
    pipeline without obvious blame attribution. Duplicating ~30 lines of
    preconditions + multi-process driver per Makefile is the cheaper
    failure mode.
  - Pipelines genuinely differ per phase (handwritten goes through
    air-translate-to-llvm + GPU compile; air_rank prepends
    air-rank-to-mgpu; air_alloc will add air-symmetric-alloc-to-mgpu;
    etc.). One unified case statement would already be hard to read by
    phase 4.

Verified on rad-mi325x-1 (2x MI325X), all 5 variants PASS:
  - make -C test/gpu/multi_gpu/handwritten INPUT={cacheline,atomic,allgather}
  - make -C test/gpu/multi_gpu/air_rank INPUT={cacheline,allgather}

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>