[Lane 2 · kotlin-core] NotImplemented stubs — Tensor.sliceCopy {} via CopyMaterializationStrategy #1415

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

Lane: 2 · Kotlin core
Skill needed: Kotlin. No tensor-storage internals needed — the materializer already exists.
Size: s (a few hours)
Blocked by: nothing

What to do

1. Tensor.sliceCopy {} in skainet-lang/skainet-lang-core/src/commonMain/kotlin/sk/ainet/lang/tensor/TensorSlicingExtensions.kt throws NotImplementedError.
2. Implement it by building the view exactly like sliceView {} in the same file and passing it to CopyMaterializationStrategy.materialize(view) (.../lang/tensor/CopyMaterializationStrategy.kt).
3. Add a test that mutating the copy does not change the source.
4. Delete the file's line from config/stub-allowlist.txt; run ./gradlew :skainet-lang:skainet-lang-core:jvmTest checkUnmarkedStubs.

Acceptance

• sliceCopy returns an independent tensor with the sliced values
• checkUnmarkedStubs passes without the allowlist entry
• 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