Extract shared stub LLM from benchmark-quality

[CalebisGross]

Summary

Move semanticStubProvider, vocabulary map, bowEmbedding(), and all stub*Response() generators from cmd/benchmark-quality/stubllm.go to a shared package at internal/testutil/stubllm/. Update cmd/benchmark-quality/ imports to use the shared package.

Why

The lifecycle test suite (#248) needs the same deterministic stub LLM. Extracting it avoids duplication and keeps both binaries in sync.

Scope

• Create internal/testutil/stubllm/ package
• Move: semanticStubProvider, vocabulary, bowEmbedding(), bowDims, all stub*Response() functions
• Export the necessary types/functions
• Update cmd/benchmark-quality/ to import from shared package
• No behavior change — pure code extraction

Test plan

• make test passes
• ./bin/mnemonic benchmark-quality produces identical output before and after
• make check passes (fmt + vet)

Part of #248