feat(explorer): add optional Git line statistics

[richardgill]

Adds opt-in Git line statistics to the Explorer.

• works across all Explorer comparison modes:
  • working tree status
  • a revision against the working tree
  • two revisions against each other
• handles staged, unstaged, renamed, deleted, binary, conflicting, and optionally bounded untracked files
• preserves path filtering when calculating statistics
• exposes per-file and aggregate folder/group statistics to the file, folder, and group formatter functions
• includes sensible default formatter functions:
  • right-aligned, truncatable file statistics
  • aggregate group totals

Screenshots

Before

require("codediff").setup({
  explorer = {
    width = 56,
    view_mode = "tree",
    flatten_dirs = false,
  },
})

After - disabled by default

require("codediff").setup({
  explorer = {
    width = 56,
    view_mode = "tree",
    flatten_dirs = false,
    -- line_stats is omitted because it is disabled by default.
  },
})

After - enabled with default formatters

require("codediff").setup({
  explorer = {
    width = 56,
    view_mode = "tree",
    flatten_dirs = false,
    line_stats = {
      enabled = true,
      count_untracked = true,
      max_untracked_bytes = 1024 * 1024,
    },
  },
})

After - custom file, folder, and group formatters

local default_formatters = require("codediff.ui.explorer.formatters")

require("codediff").setup({
  explorer = {
    width = 56,
    view_mode = "tree",
    flatten_dirs = false,
    line_stats = {
      enabled = true,
      count_untracked = true,
      max_untracked_bytes = 1024 * 1024,
    },
    formatters = {
      file = function(ctx)
        local layout = default_formatters.file(ctx)
        local right = {}

        if ctx.stats then
          local segments = ctx.stats.binary and {
            { text = "binary ", hl = "CodeDiffExplorerStatBinary" },
          } or {
            { text = "↑" .. ctx.stats.insertions, hl = "CodeDiffExplorerStatInsertions" },
            { text = " ↓" .. ctx.stats.deletions .. " ", hl = "CodeDiffExplorerStatDeletions" },
          }
          right[#right + 1] = {
            segments = segments,
            truncate_priority = 3,
          }
        end

        right[#right + 1] = {
          segments = {
            { text = ctx.status, hl = ctx.status_hl },
            { text = string.rep(" ", ctx.status_right_margin), hl = "Normal" },
          },
        }
        layout.right = right
        return layout
      end,

      folder = function(ctx)
        local layout = default_formatters.folder(ctx)
        if not ctx.stats then
          return layout
        end

        layout.right = {
          {
            segments = {
              { text = "↑" .. ctx.stats.insertions, hl = "CodeDiffExplorerStatInsertions" },
              { text = " ↓" .. ctx.stats.deletions, hl = "CodeDiffExplorerStatDeletions" },
            },
          },
        }
        return layout
      end,

      group = function(ctx)
        return {
          left = {
            {
              segments = { { text = " " .. ctx.label, hl = "CodeDiffExplorerTreeGroup" } },
              truncate_priority = 1,
            },
          },
          right = {
            {
              segments = {
                { text = ctx.file_count .. " files · ", hl = "CodeDiffExplorerStatFiles" },
                { text = "↑" .. ctx.stats.insertions, hl = "CodeDiffExplorerStatInsertions" },
                { text = " ↓" .. ctx.stats.deletions, hl = "CodeDiffExplorerStatDeletions" },
              },
            },
          },
          min_gap = 2,
        }
      end,
    },
  },
})

Supercedes part of #448.

Fixes #181

[esmuellert]

Reviewed the diff top-to-bottom: well-scoped, opt-in default, non-breaking (base git API untouched, new highlight groups use default = true, formatter contexts remain additive with stats = nil when disabled). Tests are thorough — 8 real-git integration tests + 5 UI tests covering staged/unstaged/rename/deletion/binary/conflicts/pathspec/disabled-parity/error paths.

Pushed one small refactor commit on top: split explorer/formatters.lua into a explorer/formatters/ folder module (init, common, stats, file, folder, group) so the line-stats bits and the always-active row helpers are visible from the directory listing. Pure code motion; public require path preserved; all affected specs still green.

Nice work — thanks for the clean implementation and the great test coverage.