Drop vestigial cpu dep from skainet-compile-hlo jvmMain (#472)

[michalharakal]

Closes #472.

Summary

Removes an unused `implementation(project(":skainet-backends:skainet-backend-cpu"))` from `skainet-compile/skainet-compile-hlo/build.gradle.kts`'s `jvmMain.dependencies` block. `HloGenerator` records traces with `VoidTensorOps()` from `skainet-lang-core` and the jvmMain Kotlin sources contain zero imports from `sk.ainet.backend.cpu.*` — the dependency was dead.

Why this is the right first "migration" in the P0-2 track

Grepping every `build.gradle.kts` in the repo for `skainet-backend-cpu` showed that every production-code dependency on the CPU backend is either a test source set (tests actually execute ops), an application runtime, or the BOM — with one exception: `skainet-compile-hlo`'s jvmMain, where the dep was declared but unused. The originally-scoped candidate (`skainet-compile-json`) turned out not to be migratable: its CPU dep is in `jvmTest`, where tests genuinely need a real execution context, and `skainet-backend-api` is interface-only.

So the useful first step in P0-2 isn't "migrate a consumer from CPU to api" — it's removing the one place where CPU is on a production classpath without being referenced. That strictly shrinks the transitive closure of anything pulling in `compile-hlo` and demonstrates that `skainet-lang-core` alone is enough for the production HLO export path.

Test plan

• `:skainet-compile:skainet-compile-hlo:compileKotlinJvm` green
• `:skainet-compile:skainet-compile-hlo:jvmTest` green (commonTest still has the CPU dep where tests need it)
• `:skainet-compile:skainet-compile-hlo:generateHlo` runs cleanly and prints its expected `--model is required` usage message — no `ClassNotFoundException` for `VoidTensorOps` or anything else
• CI: full multiplatform build

Out of scope

• Adding `skainet-backend-api` as a replacement dep — the code already only references types from `skainet-lang-core`, so a re-export would be pointless.
• Migrating the remaining test-source-set consumers off `skainet-backend-cpu`. Those deps are legitimate.
• Scaffolding a second backend to prove `backend-api` is actually sufficient. That's a better follow-up than another dep swap and should be a separate roadmap item.

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