test: add decoration provider, integration, and neogit spec files (#134)

## Problem

Regressions #119 and #120 showed the test suite had no coverage of the
decoration provider cache pipeline, no end-to-end pipeline tests from
buffer content to extmarks, and no Neogit-specific integration tests.

## Solution

Adds three new spec files (28 new tests, 316 total):

- `spec/decoration_provider_spec.lua` — indirect cache pipeline tests
via `_test` table: `ensure_cache` population, content fingerprint guard,
`pending_clear` semantics, BufWipeout cleanup
- `spec/integration_spec.lua` — full pipeline: diff buffer → `attach` →
extmarks; verifies `DiffsAdd`/`DiffsDelete` on correct lines, treesitter
captures, multi-hunk coverage
- `spec/neogit_integration_spec.lua` — `neogit_disable_hunk_highlight`
behavior, NeogitStatus/NeogitDiffView attach and cache population,
parser neogit filename patterns

Depends on #133.

Closes #122

spec/neogit_integration_spec.lua

@@ -0,0 +1,126 @@
+require('spec.helpers')
+
+vim.g.diffs = { neogit = true }
+
+local diffs = require('diffs')
+local parser = require('diffs.parser')
+
+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('neogit_integration', function()
+  describe('neogit_disable_hunk_highlight', function()
+    it('sets neogit_disable_hunk_highlight on NeogitStatus buffer after attach', function()
+      local bufnr = create_buffer({
+        'modified   test.lua',
+        '@@ -1,1 +1,2 @@',
+        ' local x = 1',
+        '+local y = 2',
+      })
+      vim.api.nvim_set_option_value('filetype', 'NeogitStatus', { buf = bufnr })
+      diffs.attach(bufnr)
+
+      assert.is_true(vim.b[bufnr].neogit_disable_hunk_highlight)
+
+      delete_buffer(bufnr)
+    end)
+
+    it('does not set neogit_disable_hunk_highlight on non-Neogit buffer', function()
+      local bufnr = create_buffer({})
+      vim.api.nvim_set_option_value('filetype', 'git', { buf = bufnr })
+      diffs.attach(bufnr)
+
+      assert.is_not_true(vim.b[bufnr].neogit_disable_hunk_highlight)
+
+      delete_buffer(bufnr)
+    end)
+  end)
+
+  describe('NeogitStatus buffer attach', function()
+    it('populates hunk_cache for NeogitStatus buffer with diff content', function()
+      local bufnr = create_buffer({
+        'modified   hello.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+        ' return M',
+      })
+      vim.api.nvim_set_option_value('filetype', 'NeogitStatus', { buf = bufnr })
+      diffs.attach(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      assert.is_not_nil(entry)
+      assert.is_table(entry.hunks)
+      assert.are.equal(1, #entry.hunks)
+      assert.are.equal('hello.lua', entry.hunks[1].filename)
+      delete_buffer(bufnr)
+    end)
+
+    it('populates hunk_cache for NeogitDiffView buffer', function()
+      local bufnr = create_buffer({
+        'new file   newmod.lua',
+        '@@ -0,0 +1,2 @@',
+        '+local M = {}',
+        '+return M',
+      })
+      vim.api.nvim_set_option_value('filetype', 'NeogitDiffView', { buf = bufnr })
+      diffs.attach(bufnr)
+      local entry = diffs._test.hunk_cache[bufnr]
+      assert.is_not_nil(entry)
+      assert.is_table(entry.hunks)
+      assert.are.equal(1, #entry.hunks)
+      delete_buffer(bufnr)
+    end)
+  end)
+
+  describe('parser neogit patterns', function()
+    it('detects renamed prefix via parser', function()
+      local bufnr = create_buffer({
+        'renamed   old.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+        ' return M',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+      assert.are.equal(1, #hunks)
+      assert.are.equal('old.lua', hunks[1].filename)
+      delete_buffer(bufnr)
+    end)
+
+    it('detects copied prefix via parser', function()
+      local bufnr = create_buffer({
+        'copied   orig.lua',
+        '@@ -1,2 +1,3 @@',
+        ' local M = {}',
+        '+local x = 1',
+        ' return M',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+      assert.are.equal(1, #hunks)
+      assert.are.equal('orig.lua', hunks[1].filename)
+      delete_buffer(bufnr)
+    end)
+
+    it('detects deleted prefix via parser', function()
+      local bufnr = create_buffer({
+        'deleted   gone.lua',
+        '@@ -1,2 +0,0 @@',
+        '-local M = {}',
+        '-return M',
+      })
+      local hunks = parser.parse_buffer(bufnr)
+      assert.are.equal(1, #hunks)
+      assert.are.equal('gone.lua', hunks[1].filename)
+      delete_buffer(bufnr)
+    end)
+  end)
+end)