[Bug] Communication plans can be reused across incompatible Buffer contexts

[morluto]

Problem

MoonEPCommPlan stores routing and layout decisions produced for one Buffer
context, but public plan-consuming methods do not verify that a supplied plan
belongs to that Buffer.

The plan contains:

• dst values encoded as destination_rank * NvS + local_offset;
• duplicate offsets into rank-local NvS storage;
• expert-copy decisions indexed by rank and prefetch slot;
• zero-fill ranges for the planned expert/prefetch layout.

Buffer.dispatch, combine, prefetch_weight, and reduce_grad can receive a
plan from another Buffer. Existing checks cover only parts of the required
geometry.

Concrete combine trigger

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 combine prologue can accept this pair because NvS agrees. The
kernel then uses Buffer B's N=6 iteration domain while reading Buffer A's
two-entry dst. Equal NvS therefore does not prove that the plan's iteration
domain is compatible.

A foreign prefetch plan has a second trigger: a plan produced with E=8 may
contain expert ID 7. Reusing it with an E=4 Buffer can pass the existing
experts_to_copy shape checks when B agrees, but the expert ID is outside the
consumer's four-expert tensor.

These are source-derived trigger paths.

Required invariant

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

Before synchronization or kernel launch:

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

Context ownership also establishes process-group rank ordering, device
ownership, VMM layout, and barrier-resource identity. A dimensional signature
alone cannot establish those facts.

Expected behavior

• Plans remain reusable by the Buffer that created them.
• plan.clone() preserves ownership and remains reusable.
• A plan created by another Buffer raises a clear exception before CUDA work.
• A same-owner plan with an incompatible stored signature is rejected.
• Kernel ABI, tensor layout, and planning semantics remain unchanged.

Compatibility

MoonEPCommPlan is public. A provenance field should therefore be private,
appended with a construction default, and excluded from repr/equality.
Manually constructed or deserialized plans without live Buffer ownership
should be rejected explicitly rather than treated as portable.