fix(pdf): preflight content before parsing

[McanKul]

Summary

• bound content operations, operands, recursive containers, strings, and inline-image dictionaries before lopdf parses them
• sanitize only inline-image payloads whose boundary is proven by length, decoded size, or a supported filter terminator
• fail closed for malformed, oversized, or unprovable inline-image content, including DCT payload boundaries
• keep the M4 classifier read-only; no UI, mutation, release, or main-branch changes

Verification

• cargo test --lib source_content — 63 passed
• cargo test --lib — 320 passed
• cargo clippy --lib --tests — no new warnings attributable to this change

Refs #33