ika-node: soften enforce-minimum-cpu assert to warning for localnet/dev

[t3hrache1nn]

Problem

IkaRuntimes::calculate_num_of_computations_cores() in crates/ika-core/src/runtime.rs contains assert!(total_cores_available >= 16, ...) gated on the enforce-minimum-cpu Cargo feature. Upstream ika-node's Cargo.toml includes enforce-minimum-cpu in its default features, so the assert activates implicitly on every release build unless someone passes --no-default-features.

On any dev machine with fewer than 16 physical cores (we hit this on a 12-core dev box), ika-node panics on startup:

Validator must have at least 16 CPU cores

This kills the localnet ceremony before DKG can begin. There is no localnet-friendly flag upstream today, so anyone trying to bring up a local Ika network for development on common hardware has to hand-patch the source and rebuild.

Proposed fix (this PR)

Replace assert! with if + tracing::error!. Startup continues, the operator sees a clear warning that they're running underpowered. Production validators are unaffected — they already meet the threshold so the warning never fires.

Also: replace total_cores_available - TOKIO_ALLOCATED_CORES with .saturating_sub(TOKIO_ALLOCATED_CORES).max(1) so the arithmetic doesn't underflow on machines with ≤4 cores (which would be a separate panic).

Why not just --no-default-features?

That disables more than just the CPU gate — it removes the whole feature bundle. A localnet operator shouldn't have to opt out of unrelated production features just to run on a 12-core dev laptop.

Acceptance evidence

We applied this patch on a 12-core dev box and ran a full dWallet creation ceremony end-to-end (DKG through Active) on self-hosted Ika localnet:

• dWallet ID: 0xfc2d0b3e56f3b936f8d97f9b0d2d40311315b4c95b4e5c790f67e627aebaa65e
• State: Active at v4406
• Ceremony: native ika dwallet create --curve secp256k1 (no SDK custom path)
• Cost: 0 IKA + ~0.002 SUI gas (localnet pricing)

Alternate shapes (open to maintainer preference)

1. This PR: assert! → if + tracing::error! (same behavior, warning instead of panic)
2. Stricter feature gate: keep assert!, gate it behind a new enforce-minimum-cpu-strict feature; existing enforce-minimum-cpu becomes warning-only. Lets production validators opt into the strict panic explicitly.
3. Default-feature removal: keep the assert as-is, but move enforce-minimum-cpu out of ika-node's default Cargo features. Forces production deployments to opt in; localnet works out of the box.

Happy to refactor to shape 2 or 3 if maintainers prefer — shape 1 was the minimum patch that unblocked us, but the others have different tradeoffs.