feat: per-user TOML config for default voice/speed/lang

[StuBehan]

Summary

Small config-file layer so users don't have to pass `--voice bf_emma --speed 1.1` on every invocation. Drop the values into a TOML file and they become argparse defaults; flags still override.

```toml

~/.config/stackvox/config.toml

[defaults]
voice = "bf_emma"
speed = 1.1
lang = "en-gb"
```

Resolution rules

• File location: `STACKVOX_CONFIG` env var > `$XDG_CONFIG_HOME/stackvox/config.toml` > `~/.config/stackvox/config.toml`.
• Value priority: CLI flag > config file > built-in engine default, per key.
• Tolerant: missing file → built-ins. Malformed TOML → warning logged, built-ins. `defaults` not a table → warning, built-ins. Never blocks startup.

Changes

• New `stackvox/config.py` — `Defaults` dataclass, `config_path()`, `load_defaults()`. 36 statements, 100% line coverage.
• `cli.py`: `_build_parser(defaults)` and `_add_voice_args(parser, defaults)` accept a `Defaults` object; `main()` calls `config.load_defaults()` and feeds it through.
• `pyproject.toml`: `tomli>=2.0;python_version<'3.11'` (3.11+ uses stdlib `tomllib`); `mypy` `ignore_missing_imports` extended to `tomli` so the 3.10 fallback branch type-checks on 3.11+.
• README: new "Configuration" section showing the file format.
• `tests/test_config.py`: path resolution (env + XDG + ~/.config), loading (missing/empty/full/partial/malformed/wrong-shape), and smoke tests that argparse defaults pick up the config and that explicit flags still override.

Coverage

84 tests pass (was 73). Overall coverage: 87% → 87% (new module fully tested). `stackvox/config.py` at 100%.

Test plan

• CI passes (lint, format, mypy, tests 3.10–3.13 Ubuntu, test-macos py3.12, PR-title, commit lint).
• Manually: drop a config.toml with a voice override, run `stackvox "hi"`, verify it uses the configured voice.
• Manually: pass `--voice X` on the command line, verify it overrides the config.