fix(highlight): use hl_group instead of line_hl_group for diff backgrounds

line_hl_group bg occupies a separate rendering channel from hl_group in
Neovim's extmark system, causing character-level bg-only highlights to be
invisible regardless of priority. Switching to hl_group + hl_eol ensures
all backgrounds compete in the same channel.

Also reorders priorities (Normal 198 < line bg 199 < syntax 200 < char
bg 201), bumps char-level blend alpha from 0.4 to 0.7 for visibility,
and adds debug logging throughout the intra pipeline.

lua/diffs/debug.lua

@@ -0,0 +1,68 @@
+local M = {}
+
+local ns = vim.api.nvim_create_namespace('diffs')
+
+function M.dump()
+  local bufnr = vim.api.nvim_get_current_buf()
+  local marks = vim.api.nvim_buf_get_extmarks(bufnr, ns, 0, -1, { details = true })
+  local lines = vim.api.nvim_buf_get_lines(bufnr, 0, -1, false)
+
+  local by_line = {}
+  for _, mark in ipairs(marks) do
+    local id, row, col, details = mark[1], mark[2], mark[3], mark[4]
+    local entry = {
+      id = id,
+      row = row,
+      col = col,
+      end_row = details.end_row,
+      end_col = details.end_col,
+      hl_group = details.hl_group,
+      priority = details.priority,
+      line_hl_group = details.line_hl_group,
+      number_hl_group = details.number_hl_group,
+      virt_text = details.virt_text,
+    }
+    if not by_line[row] then
+      by_line[row] = { text = lines[row + 1] or '', marks = {} }
+    end
+    table.insert(by_line[row].marks, entry)
+  end
+
+  local all_ns_marks = vim.api.nvim_buf_get_extmarks(bufnr, -1, 0, -1, { details = true })
+  local non_diffs = {}
+  for _, mark in ipairs(all_ns_marks) do
+    local details = mark[4]
+    if details.ns_id ~= ns then
+      table.insert(non_diffs, {
+        ns_id = details.ns_id,
+        row = mark[2],
+        col = mark[3],
+        end_row = details.end_row,
+        end_col = details.end_col,
+        hl_group = details.hl_group,
+        priority = details.priority,
+      })
+    end
+  end
+
+  local result = {
+    bufnr = bufnr,
+    buf_name = vim.api.nvim_buf_get_name(bufnr),
+    ns_id = ns,
+    total_diffs_marks = #marks,
+    total_all_marks = #all_ns_marks,
+    non_diffs_marks = non_diffs,
+    lines = by_line,
+  }
+
+  local state_dir = vim.fn.stdpath('state')
+  local path = state_dir .. '/diffs_debug.json'
+  local f = io.open(path, 'w')
+  if f then
+    f:write(vim.json.encode(result))
+    f:close()
+    vim.notify('[diffs.nvim] debug dump: ' .. path, vim.log.levels.INFO)
+  end
+end
+
+return M