feat(resume): add resumable processing checkpoints

[BrenoFariasdaSilva]

Description

Adds resumable processing support to libvideo2x for both interpolation and upscaling tasks.

This PR is linked to the k4yt3x/video2x-qt6#25 PR.

Interrupted jobs can reopen a validated .v2xresume workspace, replay completed frame checkpoints, and continue from the first incomplete source-frame unit.

Changes

• Adds the public resume API in libvideo2x/resume.h.
• Adds .v2xresume workspaces with versioned manifests.
• Stores per-source-frame checkpoint files and processor tail frames.
• Validates input identity, processing configuration, stream metadata, and checkpoint integrity.
• Writes manifests and checkpoint units through temporary files before replacement.
• Publishes checkpoints every 30 source frames and on cancellation or final flush.
• Adds VideoProcessor::process_resumable().
• Adds VideoProcessor::get_recovered_frames().
• Replays recovered frames before processing new frames.
• Restores the previous-frame boundary required by RIFE interpolation.
• Supports restart by deleting saved checkpoints and resetting manifest progress.
• Publishes the final output only after successful processing.
• Preserves the existing non-resumable VideoProcessor::process() API.

Frontend integration

The companion Qt frontend PR uses these APIs to inspect, restart, and resume workspaces and to report recovered progress separately.

The existing CLI continues using the non-resumable processing path.

Validation

• No resume-specific test files or CI workflow changes.
• Validation was not independently re-run while preparing this description.

Files changed

• CMakeLists.txt
• include/libvideo2x/libvideo2x.h
• include/libvideo2x/resume.h
• src/libvideo2x.cpp
• src/resume.cpp
• src/resume_session.h