Add a LICENSE file (siblings ship MIT; without one this cannot be used as a dependency)

## Summary

This repository has no `LICENSE` file (`license: NONE` via the API, checked 2026-09-11). That makes it unusable as a dependency — including the "drop it in as a header-only library" use case, since header-only still means copying source into someone else's tree.

The README describes DeepJIT as a header-only dependency to be embedded into a
consumer extension: *"DeepJIT is intended to be embedded into your extension as a
header-only dependency"*, with integration instructions that say to add
`DeepJIT/include` to the include path. Header-only still means copying and
compiling source, and this repository has no license file, so the default is
all rights reserved and the README's integration path is not available to anyone
else as written.

## It looks like an oversight rather than a policy

Every sibling project sampled in the same organisation ships MIT:

| repository | licence |
|---|---|
| DeepSeek-V3 | MIT |
| DeepGEMM | MIT |
| DeepEP | MIT |
| DeepSeek-Coder-V2 | MIT |
| DualPipe | MIT |
| DeepSeek-OCR | MIT |
| **DeepJIT** | **none** |

For completeness: `flash-mla` is the only other repository in the sampled set without a licence, so this is not a lone case — but it does look like a file that was never added rather than a deliberate reservation.

## Ask

Add the same MIT text the siblings carry (a `LICENSE` file at the root is enough; an `SPDX-License-Identifier: MIT` header in the sources makes it machine-readable if you want both).

## Impact until then

Read-only reference: the design and the kernels can be read and learned from, but nothing can be vendored or redistributed, so no downstream project can build on this — which seems the opposite of the intent for a set of JIT kernel helpers.