[Fix] Enforce Buffer ownership for communication plans

[morluto]

Fixes #8.

Summary

Tie saved communication plans to the Buffer context that produced them and
reject incompatible plan reuse before synchronization or CUDA launch.

Buffer A --planning--> Plan(marker A, signature A)
                              |
                 +------------+------------+
                 |                         |
             Buffer A                  Buffer B
          marker matches            marker differs
                 |                         |
              accepted             reject before CUDA

Problem

MoonEPCommPlan contains routing and layout decisions interpreted relative to
its originating Buffer:

• dst uses destination_rank * NvS + local_offset;
• duplicate offsets address rank-local NvS storage;
• expert-copy decisions depend on R, E, and B;
• combine iteration depends on N and K;
• process-group ordering, device ownership, VMM mappings, and barriers belong
  to one context.

Existing checks validate only parts of this geometry.

A concrete incompatible pair is:

Buffer A: R=2, E=4, B=1, S=2, K=1, token_padding=2
          N=2, NvS=6, len(plan.dst)=2

Buffer B: R=2, E=4, B=1, S=6, K=1, token_padding=1
          N=6, NvS=6

The existing NvS check accepts this pair because both values are six. Buffer
B can then use its six-entry combine iteration domain while reading Buffer A's
two-entry dst.

A foreign prefetch plan can similarly contain an expert ID valid under its
original E but outside the consuming Buffer's expert tensor.

Change

Each Buffer context now owns a private identity marker. Planning stores that
marker in the returned plan, and MoonEPCommPlan.clone() preserves it.

Before accepting a saved plan, the Buffer validates:

plan origin == consuming Buffer
plan {N,R,E,B,NvS,K} == Buffer {N,R,E,B,NvS,K}

The validation applies to:

• saved-plan dispatch;
• prefetch_weight;
• combine;
• reduce_grad.

Regression coverage

The regression covers three cases:

• a cloned plan is accepted by its originating Buffer;
• a plan from a second Buffer is rejected before launch;
• a same-owner plan with a modified signature is rejected before launch.

The final kernel call is replaced with a recorder because the behavior under
test is the synchronous pre-launch ownership boundary.

Compatibility

The provenance field is private, appended with a default, and excluded from
dataclass repr and equality.

Plans returned by a Buffer and their clones retain their current behavior.
Plans manually constructed, deserialized into another context, or transferred
between Buffer instances are now rejected.

There is no kernel ABI, tensor-layout, or planning-algorithm change.

Validation

• python3 -m py_compile moonep/planning.py moonep/api.py tests/test_dispatch.py
  — passed.
• CPU-backed validator harness — same-context clone accepted; foreign and
  signature-mutated plans rejected.
• Added focused regression coverage for the public dispatch boundary.