test: add decoration provider, integration, and neogit spec files

Problem: regressions #119 and #120 exposed gaps in coverage of the
decoration provider cache pipeline, the end-to-end highlight
pipeline, and Neogit-specific behavior.

Solution: add three spec files covering:
- decoration_provider_spec: cache population, fingerprint guard,
  pending_clear semantics, BufWipeout cleanup via _test table
- integration_spec: full buffer to attach to extmarks pipeline
- neogit_integration_spec: neogit_disable_hunk_highlight and
  filetype attachment

Closes #122

spec/decoration_provider_spec.lua

@@ -0,0 +1,176 @@
+require('spec.helpers')
+local diffs = require('diffs')
+
+describe('decoration_provider', function()
+  local function create_buffer(lines)
+    local bufnr = vim.api.nvim_create_buf(false, true)
+    vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, lines or {})
+    return bufnr
+  end
+
+  local function delete_buffer(bufnr)
+    if vim.api.nvim_buf_is_valid(bufnr) then
+      vim.api.nvim_buf_delete(bufnr, { force = true })
+    end
+  end
+
+  describe('ensure_cache', function()
+    it('populates hunk cache for a buffer with diff content', function()
+      local bufnr = create_buffer({
+        'M test.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local x = 1',
+        '-local y = 2',
+        '+local y = 3',
+        ' local z = 4',
+      })
+      diffs.attach(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      assert.is_not_nil(entry)
+      assert.is_table(entry.hunks)
+      assert.is_true(#entry.hunks > 0)
+      delete_buffer(bufnr)
+    end)
+
+    it('cache tick matches buffer changedtick after attach', function()
+      local bufnr = create_buffer({
+        'M test.lua',
+        '@@ -1,1 +1,2 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+      diffs.attach(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      local tick = vim.api.nvim_buf_get_changedtick(bufnr)
+      assert.are.equal(tick, entry.tick)
+      delete_buffer(bufnr)
+    end)
+
+    it('re-parses and advances tick when buffer content changes', function()
+      local bufnr = create_buffer({
+        'M test.lua',
+        '@@ -1,1 +1,2 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+      diffs.attach(bufnr)
+      local tick_before = diffs._test.hunk_cache[bufnr].tick
+      vim.api.nvim_buf_set_lines(bufnr, -1, -1, false, { '+local z = 3' })
+      diffs._test.ensure_cache(bufnr)
+      local tick_after = diffs._test.hunk_cache[bufnr].tick
+      assert.is_true(tick_after > tick_before)
+      delete_buffer(bufnr)
+    end)
+
+    it('skips reparse when fingerprint unchanged but sets pending_clear', function()
+      local bufnr = create_buffer({
+        'M test.lua',
+        '@@ -1,1 +1,2 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+      diffs.attach(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      local original_hunks = entry.hunks
+      entry.pending_clear = false
+
+      local lc = vim.api.nvim_buf_line_count(bufnr)
+      local bc = vim.api.nvim_buf_get_offset(bufnr, lc)
+      entry.line_count = lc
+      entry.byte_count = bc
+      entry.tick = -1
+
+      diffs._test.ensure_cache(bufnr)
+
+      local updated = diffs._test.hunk_cache[bufnr]
+      local current_tick = vim.api.nvim_buf_get_changedtick(bufnr)
+      assert.are.equal(original_hunks, updated.hunks)
+      assert.are.equal(current_tick, updated.tick)
+      assert.is_true(updated.pending_clear)
+      delete_buffer(bufnr)
+    end)
+
+    it('does nothing for invalid buffer', function()
+      local bufnr = create_buffer({})
+      diffs.attach(bufnr)
+      vim.api.nvim_buf_delete(bufnr, { force = true })
+      assert.has_no.errors(function()
+        diffs._test.ensure_cache(bufnr)
+      end)
+    end)
+  end)
+
+  describe('pending_clear', function()
+    it('is true after invalidate_cache', function()
+      local bufnr = create_buffer({})
+      diffs.attach(bufnr)
+      diffs._test.invalidate_cache(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      assert.is_true(entry.pending_clear)
+      delete_buffer(bufnr)
+    end)
+
+    it('is true immediately after fresh ensure_cache', function()
+      local bufnr = create_buffer({
+        'M test.lua',
+        '@@ -1,1 +1,2 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+      diffs.attach(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      assert.is_true(entry.pending_clear)
+      delete_buffer(bufnr)
+    end)
+  end)
+
+  describe('BufWipeout cleanup', function()
+    it('removes hunk_cache entry after buffer wipeout', function()
+      local bufnr = create_buffer({
+        'M test.lua',
+        '@@ -1,1 +1,2 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+      diffs.attach(bufnr)
+      assert.is_not_nil(diffs._test.hunk_cache[bufnr])
+      vim.api.nvim_buf_delete(bufnr, { force = true })
+      assert.is_nil(diffs._test.hunk_cache[bufnr])
+    end)
+  end)
+
+  describe('multiple hunks in cache', function()
+    it('stores all parsed hunks for a multi-hunk buffer', function()
+      local bufnr = create_buffer({
+        'M test.lua',
+        '@@ -1,2 +1,2 @@',
+        ' local x = 1',
+        '-local y = 2',
+        '+local y = 3',
+        '@@ -10,2 +10,3 @@',
+        ' function M.foo()',
+        '+  return true',
+        ' end',
+      })
+      diffs.attach(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      assert.is_not_nil(entry)
+      assert.are.equal(2, #entry.hunks)
+      delete_buffer(bufnr)
+    end)
+
+    it('stores empty hunks table for buffer with no diff content', function()
+      local bufnr = create_buffer({
+        'Head: main',
+        'Help: g?',
+        '',
+        'Nothing to see here',
+      })
+      diffs.attach(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      assert.is_not_nil(entry)
+      assert.are.same({}, entry.hunks)
+      delete_buffer(bufnr)
+    end)
+  end)
+end)