[Lane 2 · kotlin-core] NotImplemented stubs — ConstantByteSerializer: FP16 / BF16 / I8 external parameters #1419

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

Lane: 2 · Kotlin core
Skill needed: Kotlin; IEEE half/bfloat16 layout.
Size: s (a few hours)
Blocked by: nothing

What to do

1. ConstantByteSerializer (skainet-compile/skainet-compile-hlo/src/commonMain/kotlin/sk/ainet/compile/hlo/ConstantByteSerializer.kt) only serializes FP32/FP64/I32/I64 and throws otherwise.
2. Add FP16, BF16 and I8 little-endian serialization with tests; run ./gradlew :skainet-compile:skainet-compile-hlo:jvmTest.

Acceptance

• Round-trip tests for the three dtypes pass
• 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