Resolve LogSettings defaults from configuration

[samc24]

Prototype for #445: move hardcoded tunables into the layered configuration, using LogSettings as the proof of pattern.

• file_level and the console thresholds (warning_threshold, success_threshold, info_threshold) now default to None and are resolved in __post_init__ from a new [default.log] section in config.toml.
• Precedence: explicit argument, then DISPY_LOG__* environment variable, then ~/.config/disruption-py/user.toml, then the repo config.toml. The env and user-file layers come for free from the existing Dynaconf setup, e.g. DISPY_LOG__WARNING_THRESHOLD=2000.
• console_level is deliberately left alone since its None already means "derive the level from the shot count", and file_path is left alone since it is a computed default that needs its own design call.
• Log settings are machine-independent by construction: logging is set up before the tokamak is resolved in get_shots_data, so the default config environment is the right (and only possible) source. Per-machine log overrides are out of scope.
• No behavior change for existing callers: explicitly passed values always win, and the config defaults match the previous hardcoded values. Workers under spawn are unaffected since the settings are resolved in the main process before pickling.
• Added precedence tests to tests/test_config.py. The per-run config dump now includes the effective log section.

If this pattern looks right, the same treatment can roll out to the rest of the hardcoded values inventoried in #445.