fix: validate malformed atlas shapes before rendering

[sapochat]

Summary

• reject non-object atlas documents before field access
• validate root text, invariant, hierarchy, item-container, and item-field shapes
• return controlled validation errors instead of renderer tracebacks
• add malformed-shape regression coverage

Verification

• PYTHONPATH=src python3 -m unittest discover -s tests -v — 8 passed
• valid CLI render — passed
• independent malformed-input matrix — 187/187 invalid cases rejected controllably
• Python 3.11 and 3.14 checks — passed
• diff/privacy scans — clean
• independent review — passed

Review focus

• schema strictness and error clarity for malformed JSON