#274 feat(mtp): add --coupled-draft and --no-coupled-draft CLI options for coupled speculative sampling

描述

### Summary
Commit 1643965 introduced coupled draft sampling for the MTP drafter under sampling, activated via the `STRATA_SPEC_COUPLED=1` environment variable.

This PR adds first-class CLI options (`--coupled-draft` and `--no-coupled-draft`) to expose and control this feature directly from the command line and config files, while maintaining full backward compatibility with the `STRATA_SPEC_COUPLED` environment variable.

### Changes
- **`include/strata/core/coupled_draft.hpp`**:
  - Adds `set_coupled_draft(bool)` and `coupled_draft_state()` to allow programmatic and CLI configuration of coupled draft sampling.
- **`src/program/generate.cpp`**:
  - Adds `bool coupled_draft = strata::core::coupled_draft_env();` to `Options` (defaulting to the environment variable).
  - Adds `--coupled-draft` and `--no-coupled-draft` CLI argument parsing.
  - Updates `--help` text in `usage()`.