refactor: rename build to compile and watch to toggle in public API

Problem: the code used build/watch while the help file already
documented compile/toggle, creating a confusing mismatch.

Solution: rename M.build() to M.compile() and M.watch() to M.toggle()
in init.lua, update handler keys in commands.lua, and update the test
file to match.

lua/preview/commands.lua

@@ -1,8 +1,8 @@
 local M = {}
 
 local handlers = {
-  build = function()
-    require('preview').build()
+  compile = function()
+    require('preview').compile()
   end,
   stop = function()
     require('preview').stop()
@@ -10,8 +10,8 @@ local handlers = {
   clean = function()
     require('preview').clean()
   end,
-  watch = function()
-    require('preview').watch()
+  toggle = function()
+    require('preview').toggle()
   end,
   open = function()
     require('preview').open()

lua/preview/init.lua

@@ -39,10 +39,10 @@
 
 ---@class preview
 ---@field setup fun(opts?: table)
----@field build fun(bufnr?: integer)
+---@field compile fun(bufnr?: integer)
 ---@field stop fun(bufnr?: integer)
 ---@field clean fun(bufnr?: integer)
----@field watch fun(bufnr?: integer)
+---@field toggle fun(bufnr?: integer)
 ---@field open fun(bufnr?: integer)
 ---@field status fun(bufnr?: integer): preview.Status
 ---@field statusline fun(bufnr?: integer): string
@@ -144,7 +144,7 @@ function M.build_context(bufnr)
 end
 
 ---@param bufnr? integer
-function M.build(bufnr)
+function M.compile(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
   local name = M.resolve_provider(bufnr)
   if not name then
@@ -176,7 +176,7 @@ function M.clean(bufnr)
 end
 
 ---@param bufnr? integer
-function M.watch(bufnr)
+function M.toggle(bufnr)
   bufnr = bufnr or vim.api.nvim_get_current_buf()
   local name = M.resolve_provider(bufnr)
   if not name then