docs: add missing --hflip and --vflip options to README

[ogrew]

Summary

Update README.md to include the missing --hflip and --vflip options that are already implemented but were not documented.

Problem

The --hflip (horizontal flip) and --vflip (vertical flip) options were:

• ✅ Implemented in the codebase (src/options.lisp, src/ffmpeg.lisp)
• ✅ Working and generating proper output filenames (_HFlip.mp4, _VFlip.mp4)
• ✅ Documented in the help text (src/help.lisp)
• ❌ Missing from the main README documentation

This created a discrepancy between the actual functionality and user-facing documentation.

Changes Made

1. Added Usage Examples

# Flip video horizontally (left-right mirror)
visp --input sample.mp4 --hflip

# Flip video vertically (upside-down)
visp --input sample.mp4 --vflip

2. Updated Options Table

Option	Argument	Description
--hflip	(flag)	Flip video horizontally (left-right mirror).
--vflip	(flag)	Flip video vertically (upside-down).

3. Added Output Filename Examples

• sample_HFlip.mp4
• sample_VFlip.mp4

Implementation Details

These options work by:

• Parsing: Recognized as boolean flags in parse-args-to-options
• Processing: Applied as ffmpeg video filters (hflip, vflip)
• Naming: Automatically append _HFlip or _VFlip to output filenames
• Compatibility: Can be combined with other video processing options

Benefits

• Complete documentation - README now accurately reflects all implemented features
• Better user experience - Users can discover and use these video manipulation options
• Consistency - Documentation matches implementation and help text
• Professional presentation - No missing features in user-facing docs

Test plan

• Verify README formatting renders correctly
• Test that the documented options work as described:

  visp --input test.mp4 --hflip --dry-run
  visp --input test.mp4 --vflip --dry-run

• Confirm output filename generation includes the expected suffixes

This documentation update requires no code changes and simply brings the README in line with existing functionality.

🤖 Generated with Claude Code