perf: enable wasmtime disk-backed compilation cache

[matchai]

Summary

• Enable wasmtime's built-in cache feature on starship-runtime and starship-bench
• Add create_engine() that configures Cache::from_file(None) (platform default cache dir)
• Replace all Engine::default() call sites with create_engine()

Why

plugin_load benchmarks at 31.1ms — 3.7x over the 8.33ms (120fps) budget. The entire cost is Cranelift recompiling the same WASM bytes from scratch on every Module::new() call. With the cache enabled, compiled machine code is persisted to disk and deserialized on subsequent loads, skipping compilation entirely.

Cache invalidation

The cache key is a SHA-256 hash of wasm bytes + compiler target + engine config + wasmtime version. It's stored under a modules/wasmtime-<VERSION>/ subdirectory, so a wasmtime upgrade automatically busts it. Writes are atomic (temp file + rename).

Expected impact

• First load (cold cache): ~31ms (unchanged)
• Subsequent loads (cache hit): <1ms
• Production: Daemon restart skips compilation for unchanged plugins

[codspeed-hq]

Merging this PR will improve performance by ×2.5

⚡ 2 improved benchmarks
✅ 18 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	WallTime	plugin_load	31.1 ms	19.6 ms	+58.5%
⚡	Memory	plugin_load	9.5 MB	3.8 MB	×2.5

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_{Comparing perf/wasmtime-cache (1d53f3c) with main (b7011ae)}