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.

lua/diffs/init.lua

@@ -38,6 +38,8 @@
 
 ---@class diffs.NeogitConfig
 
+---@class diffs.GitsignsConfig
+
 ---@class diffs.ConflictKeymaps
 ---@field ours string|false
 ---@field theirs string|false
@@ -62,6 +64,7 @@
 ---@field highlights diffs.Highlights
 ---@field fugitive diffs.FugitiveConfig|false
 ---@field neogit diffs.NeogitConfig|false
+---@field gitsigns diffs.GitsignsConfig|false
 ---@field conflict diffs.ConflictConfig
 
 ---@class diffs
@@ -141,6 +144,7 @@ local default_config = {
   },
   fugitive = false,
   neogit = false,
+  gitsigns = false,
   conflict = {
     enabled = true,
     disable_diagnostics = true,
@@ -591,6 +595,10 @@ local function init()
     opts.neogit = {}
   end
 
+  if opts.gitsigns == true then
+    opts.gitsigns = {}
+  end
+
   vim.validate('debug', opts.debug, function(v)
     return v == nil or type(v) == 'boolean' or type(v) == 'string'
   end, 'boolean or string (file path)')
@@ -601,6 +609,9 @@ local function init()
   vim.validate('neogit', opts.neogit, function(v)
     return v == nil or v == false or type(v) == 'table'
   end, 'table or false')
+  vim.validate('gitsigns', opts.gitsigns, function(v)
+    return v == nil or v == false or type(v) == 'table'
+  end, 'table or false')
   vim.validate('extra_filetypes', opts.extra_filetypes, 'table', true)
   vim.validate('highlights', opts.highlights, 'table', true)
 
@@ -990,6 +1001,12 @@ function M.get_conflict_config()
   return config.conflict
 end
 
+---@return diffs.HunkOpts
+function M.get_highlight_opts()
+  init()
+  return { hide_prefix = config.hide_prefix, highlights = config.highlights }
+end
+
 local function process_pending_clear(bufnr)
   local entry = hunk_cache[bufnr]
   if entry and entry.pending_clear then