[tyza66]

Fixes #200.

The bug

A spec that does not parse escaped as a raw yaml.scanner.ScannerError, so
lc status and lc materialize --check tracebacked. plan._validate is the
gate that turns a bad spec into ASTRA's own errors, but it only catches what
validation reports — a file that is not YAML at all fails inside
astra.helpers.load_yaml before there is anything to validate, so the
ScannerError escapes the gate and the CLI, which only translates a
ProjectError at the _EngineErrorGroup boundary. lc materialize masked it
only because the dirty-tree refusal fires first.

The fix

One try/except yaml.YAMLError around the problems construction and the
universe loop in plan._validate, setting problems = [str(exc)]. The parse
error joins the same list the gate already renders, so the existing header and
two-space indentation carry it and no new rendering is added:

Error: .../astra.yaml does not validate, so there is nothing to materialize from it:
  mapping values are not allowed here
  in ".../astra.yaml", line 6, column 22

YAML's own error already names the file, the line and the column, which is what
the issue asked for, so it is reused verbatim rather than rebuilt from
problem_mark. Universes are covered for free: validate_universe_file loads
both the analysis and the universe, so every file is parsed inside the gate,
and the load_yaml calls build makes afterwards are then safe.

Tests

• test_a_spec_that_is_not_yaml_is_a_clean_error and
  test_a_universe_that_is_not_yaml_is_a_clean_error in tests/test_plan.py.
• test_status_on_a_spec_that_is_not_yaml_is_a_clean_error in
  tests/test_cli.py pins the CLI surface: exit 1, does not validate in the
  output, no traceback.

What I ran on Windows: ruff check src/ tests/ is clean, and mypy src/
reports no new errors (the 49 it does report are pre-existing POSIX-attribute
and fcntl/getuid failures in _sandbox_exec.py, engine/container.py and
engine/compute/, present on main too). tests/test_plan.py and
tests/test_cli.py together are 82 passed, 1 failed, where the failure is
test_a_declared_input_outside_the_project_keeps_its_absolute_path comparing a
Windows backslash path against a POSIX one — also failing on main. Across a
16-file subset, 528 passed and 72 failed, and the same 72 fail identically on
pristine main. The container and compute integration suites, plus
test_compute_local.py / test_compute_slurm.py, do not run on Windows, so
this PR's CI is their real run.

Disclosure

This change was written with AI assistance, using
Alpha.

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