feat: add public hunk navigation API

[leolaurindo]

Expose change-hunk navigation as a public API instead of forcing users to
read the internal prev_hunks state. Adds:

• inline-diff.next_hunk() / inline-diff.prev_hunk() Lua functions, jumping
  the cursor to the next/previous hunk (wrapping at either end) and
  returning the target line or nil when there are no hunks
• :InlineDiffNext / :InlineDiffPrev user commands wrapping the API

Includes 7 tests covering next/prev, wrap-around, the no-hunks case, and
clamping of deletions at line 0.

The plugin already computes and stores the current buffer's hnks interally in prev_hunks. So consumers that want hunk navigation already can do that, but need to access internal state directly (and make a wrap in accessible through init.lua). Navigation depends on

require("inline-diff.state").get(bufnr).prev_hunks

What I did was to create a set of public functions that provide stable integration without exposing state representation.

next_hunk():

• Jumps to the next hunk after the cursor
• Wraps to the first hunk after the last one
• Returns the target 1-based line
• Returns nil when the buffer is disabled or has no hunks

prev_hunk() behaves symmetrically and wraps to the final hunk.

The commands are thin wrappers around the public API.

Testing

Added tests covering:

• Next-hunk navigation
• Previous-hunk navigation
• Wrap-around in both directions
• No-hunk behavior
• Deletions anchored before line 1

Full test suite passes:
Success: 23
Failed: 0
Errors: 0

The combined demo branch is available at:
https://github.com/leolaurindo/inline-diff.nvim/tree/dev/al