fix(pdf): fail closed on unsafe content streams

[McanKul]

Summary

• Replace the classifier's decode-or-raw fallback with a bounded decoder for Flate, ASCIIHex, ASCII85, and short filter chains.
• Reject corrupt streams, predictors, external-file filters, unsupported filters, and output above the 32 MB stream cap with explicit errors.
• Reject graphics-state nesting above 64 levels instead of silently ignoring extra q operators.
• Keep this change read-only: no Tauri command, UI, mutation, or save path.

LZW remains intentionally unsupported in this research classifier and fails closed as UNSUPPORTED_FILTER.

Refs #33

Validation

• cargo check --manifest-path src-tauri/Cargo.toml
• cargo test --manifest-path src-tauri/Cargo.toml --lib source_content -- --nocapture (41 passed)
• rustfmt --edition 2021 --check src-tauri/src/pdf_engine/source_content_decode.rs
• git diff --check

Privacy checklist

• PDF data remains local.
• No source or destination file is written by the classifier.
• Filter errors never fall back to interpreting encoded bytes as PDF operators.