refactor: dedupe recipe launch-command extra-args into shared serializer

[OnlyTerp]

Summary

Extract the byte-identical launch-command extra-args logic that was duplicated between the frontend recipe preview and the controller launch builder into a single shared/command-builder.ts. Behavior-preserving.

Changes

• New shared/command-builder.ts: internal-key sets, JSON-string keys, and the extra-args serialization, parameterized per-side via an options object (false-boolean handling, shell quoting, JSON normalization).
• controller/src/modules/engines/process/backend-builder.ts and frontend/src/features/recipes/recipe-command.ts consume it. Divergent / security behavior is kept per-side (model-runtime-default heuristics, VLLM_STUDIO_ALLOW_CUSTOM_LAUNCH_COMMAND env-gating, binary path resolution, the enable_expert_parallelism false-exception).

Verification

• Before/after equivalence check: emitted launch argv is byte-identical across representative recipes (string / number / array / object / JSON-string speculative_config / boolean values, internal-key filtering, the enable_expert_parallelism exception, llama_bin security rejection, empty strings).
• cd controller && bun run typecheck → clean; bun test src/shared/command-builder.test.ts → 24 pass; frontend typecheck + build → clean.

Non-goals

• No dry-run endpoint; the preview stays offline/string-formatted exactly as before.