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/parser.lua

@@ -353,6 +353,8 @@ function M.parse_buffer(bufnr)
   return hunks
 end
 
+M.get_lang_from_ft = get_lang_from_ft
+
 M._test = {
   ft_lang_cache = ft_lang_cache,
 }