LICENSE: split third-party notices into NOTICE so GitHub detects MIT

GitHub's licensee needs ~98% similarity to a known license body. The MIT
text had the vendored-jpeg-encoder note, the "compiler is fetched at
runtime, not redistributed" note, and the ElectroMage non-affiliation
disclaimer appended after it, which dropped the match below threshold.
The repo reported license "other" / NOASSERTION and the About sidebar
showed a bare "License" link instead of "MIT license".

LICENSE is now the unmodified MIT body. The notices move verbatim into
NOTICE, and the README points at both.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PZnUXRsLLnt2A7pR3X4Luj