fix(cli): handle non-UTF-8 source files

[xuu33030]

Summary

Handle invalid UTF-8 and file-read failures before linting or executing a .dmai source file. Invalid encodings now produce a concise Maithili message and dedicated exit code instead of a raw Python traceback.

Motivation / Audit Reference

Closes #27.

Test Evidence

• pytest -v --cov=maithili_dsl --cov-report=term-missing — 162 passed, 95.00% total coverage; cli.py 97%
• New CLI and security regression tests cover invalid UTF-8 input
• All example programs smoke-tested; error.dmai failed as expected
• python -m maithili_dsl --version reports 0.4.0
• Real subprocess verification returned exit code 7, printed the Maithili UTF-8 error, and emitted no traceback

Security Considerations

The file is still read before any linting, transpilation, import validation, or execution. Malformed input now fails closed at that boundary. The generic OSError message does not expose local path or exception details.

Breaking Changes

None. Valid UTF-8 programs retain their existing behavior and exit codes. Invalid encodings now have the documented non-zero EXIT_BAD_ENCODING code instead of an uncaught exception.

Checklist

• CHANGELOG.md updated (Unreleased section)
• Version bumped if this is a release PR (not applicable)
• No new runtime dependencies added
• User-visible error behavior documented in CHANGELOG.md