feat: add process runner and status indicator

Implement runner module for spawning and managing watchexec child processes
via vim.fn.jobstart with stdout/stderr piping and ANSI stripping. Add
indicator module for displaying a small non-focusable float that shows the
last command outcome (success/failure) when the main window is hidden.

lua/watchexec/indicator.lua

@@ -0,0 +1,193 @@
+---@brief [[
+--- watchexec.nvim indicator module.
+--- Shows a small non-focusable float indicating the last command outcome
+--- while the main output window is hidden and a job is running.
+---@brief ]]
+
+---@diagnostic disable: need-check-nil
+
+local config = require("watchexec.config")
+
+local M = {}
+
+local ns = vim.api.nvim_create_namespace("watchexec-indicator-highlights")
+
+local state = {
+  win = nil,
+  buf = nil,
+  ---@type "success"|"failure"|nil
+  last_outcome = nil,
+  waiting_for_outcome = false,
+}
+
+---@return integer
+local function create_buf()
+  local buf = vim.api.nvim_create_buf(false, true)
+  vim.api.nvim_set_option_value("bufhidden", "wipe", { buf = buf })
+  vim.api.nvim_set_option_value("modifiable", true, { buf = buf })
+  vim.api.nvim_buf_set_lines(buf, 0, -1, false, { "  " })
+  vim.api.nvim_set_option_value("modifiable", false, { buf = buf })
+  return buf
+end
+
+---@return integer, integer
+local function calculate_position()
+  local cfg = config.get().indicator
+  local width = cfg.width or 2
+  local height = cfg.height or 1
+
+  if cfg.position == "bottom-left" then
+    return vim.o.lines - height - 3, 0
+  elseif cfg.position == "bottom-right" then
+    return vim.o.lines - height - 3, vim.o.columns - width
+  elseif cfg.position == "top-left" then
+    return 0, 0
+  elseif cfg.position == "top-right" then
+    return 0, vim.o.columns - width
+  end
+
+  return vim.o.lines - height - 3, 0
+end
+
+local function close_float()
+  local win = state.win
+
+  if win and vim.api.nvim_win_is_valid(win) then
+    vim.api.nvim_win_close(win, true)
+  end
+
+  state.win = nil
+end
+
+local function create_float()
+  local cfg = config.get().indicator
+  local cfg_width = cfg.width or 2
+  local cfg_height = cfg.height or 1
+  local buf = state.buf
+
+  if not buf or not vim.api.nvim_buf_is_valid(buf) then
+    buf = create_buf()
+    state.buf = buf
+  end
+
+  local row, col = calculate_position()
+  local hl = state.last_outcome == "success" and (cfg.success_hl or "WatchexecSuccess")
+    or (cfg.failure_hl or "WatchexecFailure")
+
+  vim.api.nvim_set_option_value("modifiable", true, { buf = buf })
+  vim.api.nvim_buf_set_lines(buf, 0, -1, false, { string.rep(" ", cfg_width) })
+  vim.api.nvim_set_option_value("modifiable", false, { buf = buf })
+  vim.api.nvim_buf_clear_namespace(buf, ns, 0, -1)
+  vim.api.nvim_buf_set_extmark(buf, ns, 0, 0, { end_col = cfg_width, hl_group = hl, hl_eol = true })
+
+  state.win = vim.api.nvim_open_win(buf, false, {
+    relative = "editor",
+    width = cfg_width,
+    height = cfg_height,
+    row = row,
+    col = col,
+    style = "minimal",
+    focusable = false,
+    noautocmd = true,
+  })
+end
+
+---Process output text from the running job.
+---Tracks command lifecycle based on configured patterns.
+---@param text string
+function M.process_output(text)
+  local cfg = config.get().indicator
+  local running_pat = cfg.patterns.running
+  local success_pat = cfg.patterns.success
+
+  for line in text:gmatch("[^\n]+") do
+    if running_pat and line:find(running_pat) then
+      if state.waiting_for_outcome then
+        state.last_outcome = "failure"
+      end
+
+      state.waiting_for_outcome = true
+    elseif success_pat and line:find(success_pat) then
+      state.last_outcome = "success"
+      state.waiting_for_outcome = false
+    elseif line:find("^%[Command ") and not (success_pat and line:find(success_pat)) then
+      state.last_outcome = "failure"
+      state.waiting_for_outcome = false
+    end
+  end
+end
+
+---Notify the indicator that the process has exited.
+---If a command was in-flight it is marked as failed.
+function M.process_exit()
+  if state.waiting_for_outcome then
+    state.last_outcome = "failure"
+    state.waiting_for_outcome = false
+  end
+end
+
+---Reset tracked outcome (e.g. when starting a new job).
+function M.reset()
+  state.last_outcome = nil
+  state.waiting_for_outcome = false
+end
+
+---Show or hide the indicator based on current state.
+---Shows when: enabled, window hidden, and outcome known.
+function M.refresh()
+  local cfg = config.get().indicator
+  local enabled = cfg.enabled ~= false
+  local window_visible = require("watchexec.window").is_visible()
+
+  if enabled and not window_visible and state.last_outcome then
+    if state.win and vim.api.nvim_win_is_valid(state.win) then
+      M.reposition()
+    else
+      create_float()
+    end
+  else
+    close_float()
+  end
+end
+
+---Recalculate the indicator position.
+function M.reposition()
+  local win = state.win
+
+  if not win or not vim.api.nvim_win_is_valid(win) then
+    return
+  end
+
+  local cfg = config.get().indicator
+  local cfg_width = cfg.width or 2
+  local cfg_height = cfg.height or 1
+  local row, col = calculate_position()
+  local hl = state.last_outcome == "success" and (cfg.success_hl or "WatchexecSuccess")
+    or (cfg.failure_hl or "WatchexecFailure")
+  local buf = state.buf
+
+  if buf and vim.api.nvim_buf_is_valid(buf) then
+    vim.api.nvim_buf_clear_namespace(buf, ns, 0, -1)
+    vim.api.nvim_buf_set_extmark(buf, ns, 0, 0, { end_col = cfg_width, hl_group = hl, hl_eol = true })
+  end
+
+  vim.api.nvim_win_set_config(win, {
+    relative = "editor",
+    width = cfg_width,
+    height = cfg_height,
+    row = row,
+    col = col,
+  })
+end
+
+---@return integer|nil
+function M.get_win()
+  return state.win
+end
+
+---@return "success"|"failure"|nil
+function M.get_last_outcome()
+  return state.last_outcome
+end
+
+return M