feat(diff): add layout = "inline" for VS Code-style unified diff

[zhao-tong]

Summary

Add a new layout = "inline" option for diff_opts that renders a VS Code-style unified inline diff - a single read-only buffer with deleted (red/strikethrough) and added (green) lines interleaved. This complements the existing "vertical" and "horizontal" two-panel diff layouts.

Closes #82

Motivation

The existing diff layouts use Neovim's built-in diffthis to show old and new content side by side. While effective, this uses significant screen real estate with two panels. An inline unified diff provides a more compact view that's familiar to VS
Code users, showing changes in context within a single buffer.

Changes

• New module lua/claudecode/diff_inline.lua — self-contained inline diff implementation with:
  • Pure diff computation via vim.diff() (indices mode)
  • Extmark-based rendering with sign column markers (+/-)
  • Resolution (accept/reject) and cleanup functions
• Dispatch in diff.lua — routes to inline module when layout = "inline" is configured; exposes shared utilities as M._ members following existing patterns
• Config/types — validates "inline" as a valid layout value
• close_all_diff_tabs — detects inline diff buffers via claudecode_inline_diff buffer variable
• Test mock — adds vim.diff mock with LCS-based hunk computation to tests/mocks/vim.lua
• 23 new tests covering diff computation, content extraction, rendering, MCP response format, config validation, and cleanup

Configuration

require("claudecode").setup({
  diff_opts = {
    layout = "inline", -- "vertical" (default), "horizontal", or "inline"
  },
})

Highlight groups are customizable:

• ClaudeCodeInlineDiffAdd — green background for added lines
• ClaudeCodeInlineDiffDelete — red background + strikethrough for deleted lines
• ClaudeCodeInlineDiffAddSign / ClaudeCodeInlineDiffDeleteSign — sign column colors

Requirements

• layout = "inline" requires Neovim >= 0.9.0 (for vim.diff())
• The plugin's base requirement remains Neovim >= 0.8.0; only inline layout needs 0.9+

Design Decisions

1. Separate module rather than inlining into diff.lua — keeps the already-large file manageable; inline diff uses fundamentally different rendering (extmarks vs diffthis)
2. layout field in diff state — enables dispatch in resolution/cleanup without re-reading config
3. Read-only buffer — inline diff is review-only; content extraction uses saved line_types array rather than parsing buffer text
4. Always vsplit — single buffer doesn't need horizontal/vertical choice

Test Plan

• luacheck lua/ tests/ passes with 0 warnings / 0 errors
• All 406 tests pass (23 new, 5 pre-existing failures unrelated to this change)
• Manual: layout = "inline" → single-window diff with colored lines
• Manual: Accept (aa) → file saved correctly, diff closes
• Manual: Deny (ad) → file unchanged, diff closes
• Manual: New file creation → all lines shown as green/added
• Manual: layout = "vertical" still works unchanged (regression check)