No way to select kernel backend; requested backend silently falls back to js

[ruvnet]

Summary

There is no supported way to select or require a specific kernel backend, and no clear error when the requested one is unavailable.

Observed

• METAHARNESS_KERNEL_BACKEND=wasm node bin/cli.js doctor → still reports js, silently.
• Nothing tells the operator that wasm/native couldn't be loaded or why.

Suggested

• Honor an env/flag (e.g. METAHARNESS_KERNEL_BACKEND=wasm|native|js).
• When the requested backend can't load, fail loudly (or warn) with the reason, instead of silently using js.
• Expose the resolved backend + the reason in kernelInfo().

[ruvnet]

For tracking: the three items here appear already implemented + tested on main (packages/kernel-js/src/index.ts):

• METAHARNESS_KERNEL_BACKEND=native|wasm|js is honored, and an invalid value throws (loadKernel()).
• A requested-but-unavailable backend fails loudly with the reason ("native" was requested but is unavailable: …) instead of silently using js.
• kernelDiagnostics() exposes { resolved, requested, reasons: { native?, wasm? } }.

Covered by packages/kernel-js/__tests__/loader.test.ts (METAHARNESS_KERNEL_BACKEND selection (GH #22) + kernelDiagnostics exposes resolved + requested + reasons).

The one remaining operator-facing nicety is surfacing kernelDiagnostics() in a user-run doctor command (today it reports the resolved backend but not the reasons higher tiers were unavailable). Happy to send that as a small follow-up if wanted; otherwise this looks closeable.