feat: add cycle_next_hunk option to disable hunk navigation wrap-around

[esmuellert]

Summary

Adds diff.cycle_next_hunk config option to control whether ]c/[c navigation wraps around when reaching the first/last hunk.

Changes

• Added diff.cycle_next_hunk option (default: true)
• When set to false, navigation stops at boundaries and shows a warning message
• Updated README with new option

Usage

require('codediff').setup({
  diff = {
    cycle_next_hunk = false, -- Stop at first/last hunk instead of cycling
  },
})

Benefits

• Users can easily know when they've reviewed all changes
• Matches behavior requested in [question] How to Disable File Cycling When Reviewing Changes #219

Closes #219