[Lane 2 · native] NotImplemented stubs — compile-json writeExportToFile on Kotlin/Native and JS/Wasm #1420

Sub-issue of #1414 ([Feature]: Close the remaining NotImplemented stubs outside the operator surface).

Lane: 2 · Kotlin core
Skill needed: Kotlin Multiplatform; kotlinx-io.
Size: s (a few hours)
Blocked by: nothing

What to do

1. writeExportToFile has a real actual only on JVM; the nine Serialization.*.kt actuals under skainet-compile/skainet-compile-json/src/*Main/ throw NotImplementedError.
2. Implement the native actuals (linux, macOS, iOS, androidNative) with kotlinx-io SystemFileSystem; for JS/Wasm either do the same where a filesystem exists (Node, WASI) or keep a clear UnsupportedOperationException and say why.
3. Update the corresponding lines in config/stub-allowlist.txt; run ./gradlew :skainet-compile:skainet-compile-json:allTests checkUnmarkedStubs (or the per-target test tasks you can run).

Acceptance

• Native targets write the export file
• Allowlist lines removed for every target that now works
• Result reported back on the parent issue

Notes

If something here turns out to be wrong or much bigger than the size label, comment on this issue or open a specific bug instead of blocking the parent.

Coverage matrix: docs/modules/ROOT/pages/reference/ops-status-matrix.adoc · How lanes work: Issue taxonomy