fix: require complete source verification

[be-student]

What & why

Supplying an empty or partial source_contents mapping could still produce a valid receipt verdict. Explicit source verification now requires coverage of every source recorded in the receipt.

Changes

• distinguish an omitted source mapping from an explicitly empty mapping
• report all missing source IDs alongside any content mismatches
• document the complete-mapping contract and add empty, partial, and combined-failure tests

Checklist

• Focused change with a clear description
• Ran format / lint / tests (see README)
• Updated docs or CHANGELOG if relevant
• Linked any related issue

Closes #10

Validation: pytest -q; ruff check .; python examples/demo_rag.py; git diff --check.