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/gitsigns.lua

@@ -0,0 +1,172 @@
+local M = {}
+
+local api = vim.api
+local fn = vim.fn
+local dbg = require('diffs.log').dbg
+
+local ns = api.nvim_create_namespace('diffs-gitsigns')
+local gs_popup_ns = api.nvim_create_namespace('gitsigns_popup')
+
+local patched = false
+
+---@param bufnr integer
+---@param src_filename string
+---@param src_ft string?
+---@param src_lang string?
+---@return diffs.Hunk[]
+function M.parse_blame_hunks(bufnr, src_filename, src_ft, src_lang)
+  local lines = api.nvim_buf_get_lines(bufnr, 0, -1, false)
+  local hunks = {}
+  local hunk_lines = {}
+  local hunk_start = nil
+
+  for i, line in ipairs(lines) do
+    if line:match('^Hunk %d+ of %d+') then
+      if hunk_start and #hunk_lines > 0 then
+        table.insert(hunks, {
+          filename = src_filename,
+          ft = src_ft,
+          lang = src_lang,
+          start_line = hunk_start,
+          prefix_width = 1,
+          quote_width = 0,
+          lines = hunk_lines,
+        })
+      end
+      hunk_lines = {}
+      hunk_start = i
+    elseif hunk_start then
+      if line:match('^%(guessed:') then
+        hunk_start = i
+      else
+        local prefix = line:sub(1, 1)
+        if prefix == ' ' or prefix == '+' or prefix == '-' then
+          if #hunk_lines == 0 then
+            hunk_start = i - 1
+          end
+          table.insert(hunk_lines, line)
+        end
+      end
+    end
+  end
+
+  if hunk_start and #hunk_lines > 0 then
+    table.insert(hunks, {
+      filename = src_filename,
+      ft = src_ft,
+      lang = src_lang,
+      start_line = hunk_start,
+      prefix_width = 1,
+      quote_width = 0,
+      lines = hunk_lines,
+    })
+  end
+
+  return hunks
+end
+
+---@param preview_winid integer
+---@param preview_bufnr integer
+local function on_preview(preview_winid, preview_bufnr)
+  local ok, err = pcall(function()
+    if not api.nvim_buf_is_valid(preview_bufnr) then
+      return
+    end
+    if not api.nvim_win_is_valid(preview_winid) then
+      return
+    end
+
+    local win = api.nvim_get_current_win()
+    if win == preview_winid then
+      win = fn.win_getid(fn.winnr('#'))
+    end
+    if win == -1 or win == 0 or not api.nvim_win_is_valid(win) then
+      return
+    end
+
+    local srcbuf = api.nvim_win_get_buf(win)
+    if not api.nvim_buf_is_loaded(srcbuf) then
+      return
+    end
+
+    local ft = vim.bo[srcbuf].filetype
+    local name = api.nvim_buf_get_name(srcbuf)
+    if not name or name == '' then
+      name = ft and ('a.' .. ft) or 'unknown'
+    end
+    local lang = ft and require('diffs.parser').get_lang_from_ft(ft) or nil
+
+    local hunks = M.parse_blame_hunks(preview_bufnr, name, ft, lang)
+    if #hunks == 0 then
+      return
+    end
+
+    local diff_start = hunks[1].start_line
+    local last = hunks[#hunks]
+    local diff_end = last.start_line + #last.lines
+
+    api.nvim_buf_clear_namespace(preview_bufnr, gs_popup_ns, diff_start, diff_end)
+    api.nvim_buf_clear_namespace(preview_bufnr, ns, diff_start, diff_end)
+
+    local opts = require('diffs').get_highlight_opts()
+    local highlight = require('diffs.highlight')
+    for _, hunk in ipairs(hunks) do
+      highlight.highlight_hunk(preview_bufnr, ns, hunk, opts)
+      for j, line in ipairs(hunk.lines) do
+        local ch = line:sub(1, 1)
+        if ch == '+' or ch == '-' then
+          pcall(api.nvim_buf_set_extmark, preview_bufnr, ns, hunk.start_line + j - 1, 0, {
+            end_col = 1,
+            hl_group = ch == '+' and '@diff.plus' or '@diff.minus',
+            priority = opts.highlights.priorities.syntax,
+          })
+        end
+      end
+    end
+
+    dbg('gitsigns blame: highlighted %d hunks in popup buf %d', #hunks, preview_bufnr)
+  end)
+  if not ok then
+    dbg('gitsigns blame error: %s', err)
+  end
+end
+
+---@return boolean
+function M.setup()
+  if patched then
+    return true
+  end
+
+  local pop_ok, Popup = pcall(require, 'gitsigns.popup')
+  if not pop_ok or not Popup then
+    return false
+  end
+
+  Popup.create = (function(orig)
+    return function(...)
+      local winid, bufnr = orig(...)
+      on_preview(winid, bufnr)
+      return winid, bufnr
+    end
+  end)(Popup.create)
+
+  Popup.update = (function(orig)
+    return function(winid, bufnr, ...)
+      orig(winid, bufnr, ...)
+      on_preview(winid, bufnr)
+    end
+  end)(Popup.update)
+
+  patched = true
+  dbg('gitsigns popup patched')
+  return true
+end
+
+M._test = {
+  parse_blame_hunks = M.parse_blame_hunks,
+  on_preview = on_preview,
+  ns = ns,
+  gs_popup_ns = gs_popup_ns,
+}
+
+return M