plugin-wasm: PluginRuntimeConfig.enable_simd=false fails initialization due to default relaxed-simd proposal

[staging-devin-ai-integration]

Summary

PluginRuntimeConfig exposes enable_simd: bool and PluginRuntime::new forwards it to wasmtime via config.wasm_simd(...). However, wasmtime enables the relaxed-simd proposal by default, which requires the base SIMD proposal. As a result, every enable_simd: false configuration — with or without enable_threads — fails initialization with:

cannot disable the simd proposal but enable the relaxed simd proposal

The public config option is therefore unusable.

Repro

let cfg = PluginRuntimeConfig {
    max_memory_bytes: 16 * 1024 * 1024,
    enable_simd: false,
    enable_threads: false,
};
assert!(PluginRuntime::new(cfg).is_err()); // currently always true

Pinned by tests::plugin_runtime_new_rejects_disabled_simd_due_to_relaxed_simd_default in https://github.com/streamer45/streamkit/blob/main/crates/plugin-wasm/src/lib.rs.

Suggested fix

Either:

• Honor enable_simd: false end-to-end by also calling config.wasm_relaxed_simd(false) (and any other dependent proposals) in PluginRuntime::new, or
• Remove the field from the public config if disabling SIMD is intentionally unsupported.

Once fixed, update plugin_runtime_new_rejects_disabled_simd_due_to_relaxed_simd_default to assert the intended contract.

Discovered by

PR #467 (Phase 3 coverage initiative — Stream B).