feat(conflict): add virtual text formatting and action lines (#101)

## Problem

Conflict resolution virtual text only showed plain "current" /
"incoming"
labels with no keymap hints. Users had no way to discover available
resolution keymaps without reading docs.

## Solution

Default virtual text labels now include keymap hints: `(current — doo)`
and
`(incoming — dot)`. A new `format_virtual_text` config option lets users
customize or hide labels entirely. A new `show_actions` option (off by
default) renders a codelens-style action line above each `<<<<<<<`
marker
listing all enabled resolution keymaps. Merge diff views also gain hunk
hints on `@@` header lines showing available keymaps.

New config fields: `conflict.format_virtual_text` (function|nil),
`conflict.show_actions` (boolean). New highlight group:
`DiffsConflictActions`.

spec/conflict_spec.lua

@@ -6,6 +6,7 @@ local function default_config(overrides)
     enabled = true,
     disable_diagnostics = false,
     show_virtual_text = true,
+    show_actions = false,
     keymaps = {
       ours = 'doo',
       theirs = 'dot',
@@ -685,4 +686,158 @@ describe('conflict', function()
       helpers.delete_buffer(bufnr)
     end)
   end)
+
+  describe('virtual text formatting', function()
+    after_each(function()
+      conflict.detach(vim.api.nvim_get_current_buf())
+    end)
+
+    it('default labels show current and incoming without keymaps', function()
+      local bufnr = create_file_buffer({
+        '<<<<<<< HEAD',
+        'local x = 1',
+        '=======',
+        'local x = 2',
+        '>>>>>>> feature',
+      })
+
+      conflict.attach(bufnr, default_config())
+
+      local extmarks = get_extmarks(bufnr)
+      local labels = {}
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].virt_text then
+          table.insert(labels, mark[4].virt_text[1][1])
+        end
+      end
+      assert.are.equal(2, #labels)
+      assert.are.equal(' (current)', labels[1])
+      assert.are.equal(' (incoming)', labels[2])
+
+      helpers.delete_buffer(bufnr)
+    end)
+
+    it('uses custom format_virtual_text function', function()
+      local bufnr = create_file_buffer({
+        '<<<<<<< HEAD',
+        'local x = 1',
+        '=======',
+        'local x = 2',
+        '>>>>>>> feature',
+      })
+
+      conflict.attach(
+        bufnr,
+        default_config({
+          format_virtual_text = function(side)
+            return side == 'ours' and 'OURS' or 'THEIRS'
+          end,
+        })
+      )
+
+      local extmarks = get_extmarks(bufnr)
+      local labels = {}
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].virt_text then
+          table.insert(labels, mark[4].virt_text[1][1])
+        end
+      end
+      assert.are.equal(2, #labels)
+      assert.are.equal(' (OURS)', labels[1])
+      assert.are.equal(' (THEIRS)', labels[2])
+
+      helpers.delete_buffer(bufnr)
+    end)
+
+    it('hides label when format_virtual_text returns nil', function()
+      local bufnr = create_file_buffer({
+        '<<<<<<< HEAD',
+        'local x = 1',
+        '=======',
+        'local x = 2',
+        '>>>>>>> feature',
+      })
+
+      conflict.attach(
+        bufnr,
+        default_config({
+          format_virtual_text = function()
+            return nil
+          end,
+        })
+      )
+
+      local extmarks = get_extmarks(bufnr)
+      local virt_text_count = 0
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].virt_text then
+          virt_text_count = virt_text_count + 1
+        end
+      end
+      assert.are.equal(0, virt_text_count)
+
+      helpers.delete_buffer(bufnr)
+    end)
+  end)
+
+  describe('action lines', function()
+    after_each(function()
+      conflict.detach(vim.api.nvim_get_current_buf())
+    end)
+
+    it('adds virt_lines when show_actions is true', function()
+      local bufnr = create_file_buffer({
+        '<<<<<<< HEAD',
+        'local x = 1',
+        '=======',
+        'local x = 2',
+        '>>>>>>> feature',
+      })
+
+      conflict.attach(bufnr, default_config({ show_actions = true }))
+
+      local extmarks = get_extmarks(bufnr)
+      local virt_lines_count = 0
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].virt_lines then
+          virt_lines_count = virt_lines_count + 1
+        end
+      end
+      assert.are.equal(1, virt_lines_count)
+
+      helpers.delete_buffer(bufnr)
+    end)
+
+    it('omits disabled keymaps from action line', function()
+      local bufnr = create_file_buffer({
+        '<<<<<<< HEAD',
+        'local x = 1',
+        '=======',
+        'local x = 2',
+        '>>>>>>> feature',
+      })
+
+      conflict.attach(
+        bufnr,
+        default_config({ show_actions = true, keymaps = { both = false, none = false } })
+      )
+
+      local extmarks = get_extmarks(bufnr)
+      for _, mark in ipairs(extmarks) do
+        if mark[4] and mark[4].virt_lines then
+          local line = mark[4].virt_lines[1]
+          local text = ''
+          for _, chunk in ipairs(line) do
+            text = text .. chunk[1]
+          end
+          assert.is_truthy(text:find('Current'))
+          assert.is_truthy(text:find('Incoming'))
+          assert.is_falsy(text:find('Both'))
+          assert.is_falsy(text:find('None'))
+        end
+      end
+
+      helpers.delete_buffer(bufnr)
+    end)
+  end)
 end)