feat: allow caller-controlled ORT loading on 0.3.74

[AnthonyRonning]

Summary

• branch directly from the exact published pdf_oxide 0.3.74 commit
• add a loader-neutral ocr-ort feature for applications that already manage ONNX Runtime linking/loading
• keep the existing ocr and gpu features backward-compatible by retaining dynamic loading
• move only OCR feature gates and documentation; parser and inference algorithms are unchanged

Validation

• cargo fmt --all -- --check
• cargo check --locked --no-default-features --features ocr-ort --lib
• cargo check --locked --no-default-features --features ocr --lib
• cargo check --locked --no-default-features --features gpu --lib
• feature-tree assertions confirm ocr-ort does not enable load-dynamic/disable-linking and legacy ocr does

This maintenance PR intentionally targets the fork branch maple-0.3.74 so Maple receives no unpublished upstream parser or dependency changes.

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