feat: gitsigns blame popup highlighting (#157)

## Problem

gitsigns' `:Gitsigns blame_line` popup shows flat
`GitSignsAddPreview`/`GitSignsDeletePreview` line highlights with basic
word-level inline diffs, but no treesitter syntax or diffs.nvim's
character-level intra-line highlighting.

## Solution

Add `lua/diffs/gitsigns.lua` which patches gitsigns' `Popup.create` and
`Popup.update` to intercept blame popups. Parses `Hunk N of M` sections
from the popup buffer, clears gitsigns' own `gitsigns_popup` namespace
on the diff region, and applies `highlight_hunk` with manual
`@diff.plus`/`@diff.minus` prefix extmarks. Uses a separate
`diffs-gitsigns` namespace to avoid colliding with the main decoration
provider.

Enabled via `vim.g.diffs = { gitsigns = true }`. Wired in
`plugin/diffs.lua` with a `User GitAttach` lazy-load retry for when
gitsigns loads after diffs.nvim. Config plumbing adds
`get_highlight_opts()` as a public getter, replacing the
`debug.getupvalue` hack used by the standalone `blame_hl.nvim` plugin.

Closes #155.

README.md

@@ -16,6 +16,7 @@ with language-aware syntax highlighting.
   word-level accuracy)
 - `:Gdiff` unified diff against any revision
 - Inline merge conflict detection, highlighting, and resolution
+- gitsigns.nvim blame popup highlighting
 - Email quoting/patch syntax support (`> diff ...`)
 - Vim syntax fallback
 - Configurable highlighiting blend & priorities
@@ -58,14 +59,15 @@ luarocks install diffs.nvim
 Do not lazy load `diffs.nvim` with `event`, `lazy`, `ft`, `config`, or `keys` to
 control loading - `diffs.nvim` lazy-loads itself.
 
-**Q: Does diffs.nvim support vim-fugitive/Neogit?**
+**Q: Does diffs.nvim support vim-fugitive/Neogit/gitsigns?**
 
-Yes. Enable it in your config:
+Yes. Enable integrations in your config:
 
 ```lua
 vim.g.diffs = {
   fugitive = true,
   neogit = true,
+  gitsigns = true,
 }
 ```
 
@@ -119,4 +121,5 @@ See the documentation for more information.
 - [`git-conflict.nvim`](https://github.com/akinsho/git-conflict.nvim)
 - [@phanen](https://github.com/phanen) - diff header highlighting, unknown
   filetype fix, shebang/modeline detection, treesitter injection support,
-  decoration provider highlighting architecture
+  decoration provider highlighting architecture, gitsigns blame popup
+  highlighting