fix(rivetkit): omit native engine path for wasm runtime

[NathanFlurry]

• Pass the npm Engine binary path only to the native runtime.
• Preserve native local-Engine startup while allowing configured WASM registries to initialize.

[railway-app]

🚅 Deployed to the actors-pr-5626 environment in rivet-frontend

Service	Status	Web	Updated (UTC)
frontend-cloud	😴 Sleeping (View Logs)	Web	Aug 31, 2026 at 5:19 am
frontend-inspector	😴 Sleeping (View Logs)	Web	Aug 31, 2026 at 5:19 am
kitchen-sink	😴 Sleeping (View Logs)	Web	Aug 31, 2026 at 5:17 am
ladle	✅ Success (View Logs)	Web	Aug 31, 2026 at 5:10 am
mcp-hub	✅ Success (View Logs)	Web	Aug 31, 2026 at 5:08 am
website	❌ Build Failed (View Logs)	Web	Aug 31, 2026 at 5:07 am

[claude]

Code Review

Reviewed this PR (fix(rivetkit): omit native engine path for wasm runtime) against the repo conventions in CLAUDE.md.

Summary: Small, well-scoped fix that gates native engine-binary-path resolution behind runtimeKind === "napi" in rivetkit-typescript/packages/rivetkit/src/registry/native.ts, so WASM registries no longer get (and do not need) a native binary path.

Findings: None survived review.

Verification performed:

• CoreRuntime["kind"] is an exhaustive "napi" | "wasm" union, so gating on === "napi" correctly covers the wasm branch without needing a third case.
• The single call site (buildRegistryWithRuntime) and both test call sites were updated to the new two-argument signature; no stale single-argument callers remain elsewhere in the repo.
• RuntimeServeConfig.engineBinaryPath is optional, so leaving it unset for the wasm path is type-safe, and the new tests toBeUndefined() assertion is correct.
• The Rust side (engine-process/src/lib.rs) independently re-reads RIVET_ENGINE_BINARY_PATH from the environment as a fallback, so omitting the JS-resolved path for wasm does not regress that override path.
• This matches the documented convention to select runtime behavior from CoreRuntime.kind rather than instanceof adapter classes.

Test coverage looks appropriate for the change (registry-constructor test updated to assert the wasm path leaves engineBinaryPath undefined while native still resolves it). No security, performance, or style concerns found.

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