Improve AEDB source shape and index scan safety

[johnny-emp]

Summary

• split large implementation/test modules and add source-shape guardrails
• replace several production panic paths with explicit error handling or fixed-array parsing
• fix secondary-index range scan capacity overflow for uncapped residual LIKE scans

Validation

• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --test source_shape
• cargo test --lib storage::index
• cargo bench --bench perf index_like_name_residual_limit_20 -- --sample-size=10

Notes

• Sanitization scan found no local identity or machine-path strings in the shipped source/test/docs paths.
• The targeted benchmark now completes after the index scan safety fix; I am not claiming broad throughput improvement from the structural cleanup alone.