feat(buffer): add scratch buffer with concealment and extmarks

Problem: need a buffer to display tasks with concealed IDs,
virtual text due dates, strikethrough for done items, and
auto-indent.

Solution: add buffer module with acwrite scratch buffer, syntax
conceal for /id/ prefixes, right-aligned due date extmarks,
header/done highlighting, indentexpr, and view toggling.

lua/todo/buffer.lua

@@ -0,0 +1,194 @@
+local config = require('todo.config')
+local store = require('todo.store')
+local views = require('todo.views')
+
+---@class todo.buffer
+local M = {}
+
+---@type integer?
+local task_bufnr = nil
+local task_ns = vim.api.nvim_create_namespace('todo')
+---@type 'category'|'priority'|nil
+local current_view = nil
+---@type todo.LineMeta[]
+local _meta = {}
+
+---@return todo.LineMeta[]
+function M.meta()
+  return _meta
+end
+
+---@return integer?
+function M.bufnr()
+  return task_bufnr
+end
+
+---@return string?
+function M.current_view_name()
+  return current_view
+end
+
+---@param bufnr integer
+local function set_buf_options(bufnr)
+  vim.bo[bufnr].buftype = 'acwrite'
+  vim.bo[bufnr].bufhidden = 'hide'
+  vim.bo[bufnr].swapfile = false
+  vim.bo[bufnr].filetype = 'todo'
+  vim.bo[bufnr].modifiable = true
+end
+
+---@param winid integer
+local function set_win_options(winid)
+  vim.wo[winid].conceallevel = 3
+  vim.wo[winid].concealcursor = 'nvic'
+  vim.wo[winid].wrap = false
+  vim.wo[winid].number = false
+  vim.wo[winid].relativenumber = false
+  vim.wo[winid].signcolumn = 'no'
+  vim.wo[winid].foldcolumn = '0'
+  vim.wo[winid].spell = false
+  vim.wo[winid].cursorline = true
+end
+
+---@param bufnr integer
+local function setup_syntax(bufnr)
+  vim.api.nvim_buf_call(bufnr, function()
+    vim.cmd([[
+      syntax clear
+      syntax match taskId /^\/\d\+\// conceal
+      syntax match taskHeader /^\S.*$/ contains=taskId
+      syntax match taskPriority /! / contained containedin=taskLine
+      syntax match taskLine /^\/\d\+\/  .*$/ contains=taskId,taskPriority
+    ]])
+  end)
+end
+
+---@param bufnr integer
+local function setup_indentexpr(bufnr)
+  vim.bo[bufnr].indentexpr = 'v:lua.require("todo.buffer").get_indent()'
+end
+
+---@return integer
+function M.get_indent()
+  local lnum = vim.v.lnum
+  if lnum <= 1 then
+    return 0
+  end
+  local prev = vim.fn.getline(lnum - 1)
+  if prev == '' or prev:match('^%S') then
+    return 0
+  end
+  return 2
+end
+
+---@param bufnr integer
+---@param line_meta todo.LineMeta[]
+local function apply_extmarks(bufnr, line_meta)
+  vim.api.nvim_buf_clear_namespace(bufnr, task_ns, 0, -1)
+  for i, m in ipairs(line_meta) do
+    local row = i - 1
+    if m.type == 'task' then
+      if m.due then
+        vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
+          virt_text = { { m.due, 'TodoDue' } },
+          virt_text_pos = 'right_align',
+        })
+      end
+      if m.status == 'done' then
+        local line = vim.api.nvim_buf_get_lines(bufnr, row, row + 1, false)[1] or ''
+        local col_start = line:find('/%d+/') and select(2, line:find('/%d+/')) + 2 or 0
+        vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, col_start, {
+          end_col = #line,
+          hl_group = 'TodoDone',
+        })
+      end
+    elseif m.type == 'header' then
+      local line = vim.api.nvim_buf_get_lines(bufnr, row, row + 1, false)[1] or ''
+      vim.api.nvim_buf_set_extmark(bufnr, task_ns, row, 0, {
+        end_col = #line,
+        hl_group = 'TodoHeader',
+      })
+    end
+  end
+end
+
+local function setup_highlights()
+  local function hl(name, opts)
+    if vim.fn.hlexists(name) == 0 or vim.tbl_isempty(vim.api.nvim_get_hl(0, { name = name })) then
+      vim.api.nvim_set_hl(0, name, opts)
+    end
+  end
+  hl('TodoHeader', { bold = true })
+  hl('TodoDue', { fg = '#888888', italic = true })
+  hl('TodoDone', { strikethrough = true, fg = '#666666' })
+  hl('TodoPriority', { fg = '#e06c75', bold = true })
+end
+
+---@param bufnr? integer
+function M.render(bufnr)
+  bufnr = bufnr or task_bufnr
+  if not bufnr or not vim.api.nvim_buf_is_valid(bufnr) then
+    return
+  end
+
+  current_view = current_view or config.get().default_view
+  local tasks = store.active_tasks()
+
+  local lines, line_meta
+  if current_view == 'priority' then
+    lines, line_meta = views.priority_view(tasks)
+  else
+    lines, line_meta = views.category_view(tasks)
+  end
+
+  _meta = line_meta
+
+  vim.bo[bufnr].modifiable = true
+  vim.api.nvim_buf_set_lines(bufnr, 0, -1, false, lines)
+  vim.bo[bufnr].modified = false
+
+  setup_syntax(bufnr)
+  apply_extmarks(bufnr, line_meta)
+end
+
+function M.toggle_view()
+  if current_view == 'category' then
+    current_view = 'priority'
+  else
+    current_view = 'category'
+  end
+  M.render()
+end
+
+---@return integer bufnr
+function M.open()
+  setup_highlights()
+  store.load()
+
+  if task_bufnr and vim.api.nvim_buf_is_valid(task_bufnr) then
+    local wins = vim.fn.win_findbuf(task_bufnr)
+    if #wins > 0 then
+      vim.api.nvim_set_current_win(wins[1])
+      M.render(task_bufnr)
+      return task_bufnr
+    end
+    vim.api.nvim_set_current_buf(task_bufnr)
+    set_win_options(vim.api.nvim_get_current_win())
+    M.render(task_bufnr)
+    return task_bufnr
+  end
+
+  task_bufnr = vim.api.nvim_create_buf(true, false)
+  vim.api.nvim_buf_set_name(task_bufnr, 'todo://')
+
+  set_buf_options(task_bufnr)
+  setup_indentexpr(task_bufnr)
+  vim.api.nvim_set_current_buf(task_bufnr)
+  set_win_options(vim.api.nvim_get_current_win())
+
+  M.render(task_bufnr)
+
+  return task_bufnr
+end
+
+return M