feat: expose navigation API (next_hunk, prev_hunk, next_file, prev_file)

[esmuellert]

Summary

Exposes navigation functions as public API, allowing users to customize navigation behavior (e.g., centering view after jumps).

Closes #217

Changes

• New file: lua/codediff/ui/view/navigation.lua - standalone navigation module
• Updated: lua/codediff/init.lua - exports next_hunk(), prev_hunk(), next_file(), prev_file()
• Refactored: lua/codediff/ui/view/keymaps.lua - uses navigation module instead of inline functions
• New test: tests/api_spec.lua - verifies API exports

Usage

local codediff = require('codediff')

-- Example: center view after each jump (from issue #217)
vim.keymap.set('n', ']c', function()
  codediff.next_hunk()
  vim.cmd('zz')
end, { desc = 'Next hunk (centered)' })

vim.keymap.set('n', '[c', function()
  codediff.prev_hunk()
  vim.cmd('zz')
end, { desc = 'Prev hunk (centered)' })

API

Function	Description
next_hunk()	Navigate to next diff hunk. Returns true if succeeded.
prev_hunk()	Navigate to previous diff hunk. Returns true if succeeded.
next_file()	Navigate to next file (or next commit in single-file history mode). Returns true if succeeded.
prev_file()	Navigate to previous file. Returns true if succeeded.