Raise a clear error when fenn.yaml is empty or not a mapping

[pcbeingused333]

Problem

Parser.load_configuration:

with open(self._config_file) as f:
    self._args = yaml.safe_load(f)
    self._args["project"] = self._config_file.stem

yaml.safe_load returns None for an empty file and a list/str/int for a top-level non-mapping, so the next line raises a bare TypeError: 'NoneType' object does not support item assignment (or list indices must be integers or slices, not str). The user just gets a stack trace into fenn internals instead of being told their config file is empty or malformed.

Both entry points hit it: App._load_configuration and fenn grid (_parse_grid).

Fix

Validate the parsed value and raise ValueError with a message that names the file and the problem, mirroring the existing _config_missing helper.

Tests

New tests/unit/test_parser.py: happy path, missing file, empty file, non-mapping. The empty/non-mapping cases raise TypeError on main and ValueError with the fix (verified: pytest tests/unit — 922 passed, ruff check/format clean).

[pcbeingused333]

The pre-commit check is red on the ty hook, but that is pre-existing on main (commit 0d1862e fails the same job, on fenn/experimental/vision/normalize.py and fenn/logging.py — neither touched here). ruff check / ruff format pass and ty check fenn/parser.py is clean; the four test jobs are the relevant ones and they are green.