PLAN/SPEC describe module coordinates and structure that no longer match the repo

[devcrocod]

Several docs are out of sync with the current module layout:

• PLAN.md refers to simdjson-kotlin-jvm-runtime-* coordinates; the real artifacts are simdjson-kotlin-jni-runtime-*
• SPEC.md calls modules :simdjson-kotlin-core/:simdjson-kotlin-benchmarks; the real names are :simdjson-kotlin/:benchmarks
• ISSUES.md still lists an old depth/maxDepth bug as open even though it's already fixed in code
• settings.gradle.kts has 3 modules where PLAN.md describes "5 runtime subprojects" (the design moved into convention plugins, which is fine, but the doc wasn't updated)

Worth a pass over PLAN.md/SPEC.md/ISSUES.md to bring them back in line with the actual repo.