docs: English-first README skeleton + CITATION.cff

[sparkainlp-x]

• README: English-first account-wide skeleton (badges incl. license + CI + research-prototype + hardware UNRUN, What it is / is NOT, Quickstart, Tests, Evidence tags, Relationship to ADR-001, Citation, License). The original French documentation is kept in full below it (headings nested under Description en français). Evidence table and known limitations unchanged in substance.
• CITATION.cff (v0.1.0, MIT, ORCID left as a commented placeholder; references the upstream ldpc library). Validated with cffconvert.

Quickstart verified in a fresh clone (Debian, g++ 14.2, CMake 3.31, default generator): build OK, smoke binary prints the expected two lines, ctest 100% passed, benchmark PASS under the 1 µs gate. I deliberately did not add the local benchmark number to the README, to avoid publishing a new metric. No code or workflow changes. PR #2 stays open (awaiting author data).