docs: produce cortex-tensor role in runtime/ownership boundary matrix (LIM-9)

[rmems]

Context

Part of org-wide boundary planning (Linear LIM-9).

cortex-tensor provides pure-Rust tensor, transformer, and MoE building blocks (zero GPU/CUDA/Julia). It is a core math primitive that hybrid-fusion and engram-parser consumers rely on.

Goal

Document owns/does-not-own, allowed deps, and how it fits vs. SNN crates (neuromod etc.) and domain apps.

Acceptance criteria

• Purpose and boundaries documented
• README updated with Scope section (consistent style)
• Link to LIM-9
• Planning only

References

• neuromod#25
• hybrid-fusion, engram-parser

Labels

documentation

Suggested branch

docs/cortex-tensor-boundary-matrix