docs: document extra_args provider field

Problem: Overriding a single flag (e.g. `-outdir=build`) required
redefining the entire `args` function, duplicating all preset defaults.

Solution: Add `extra_args` field that appends to the resolved `args`
after evaluation. Accepts a static table or a context function.

doc/preview.txt

@@ -68,6 +68,10 @@ Provider fields: ~
                                         receives a |preview.Context| and
                                         returns a string[].
 
+  {extra_args}       (string[]|function) Appended to {args} after evaluation.
+                                        Useful for adding flags to a preset
+                                        without replacing its defaults.
+
   {cwd}              (string|function)  Working directory. If a function,
                                         receives a |preview.Context|.
                                         Default: git root or file directory.

lua/preview/compiler.lua

@@ -309,6 +309,9 @@ function M.compile(bufnr, name, provider, ctx, opts)
   if provider.args then
     vim.list_extend(cmd, eval_list(provider.args, resolved_ctx))
   end
+  if provider.extra_args then
+    vim.list_extend(cmd, eval_list(provider.extra_args, resolved_ctx))
+  end
 
   log.dbg('compiling buffer %d with provider "%s": %s', bufnr, name, table.concat(cmd, ' '))
 

lua/preview/init.lua

@@ -2,6 +2,7 @@
 ---@field ft? string
 ---@field cmd string[]
 ---@field args? string[]|fun(ctx: preview.Context): string[]
+---@field extra_args? string[]|fun(ctx: preview.Context): string[]
 ---@field cwd? string|fun(ctx: preview.Context): string
 ---@field env? table<string, string>
 ---@field output? string|fun(ctx: preview.Context): string
@@ -94,6 +95,7 @@ function M.setup(opts)
     vim.validate(prefix .. '.cmd', provider.cmd, 'table')
     vim.validate(prefix .. '.cmd[1]', provider.cmd[1], 'string')
     vim.validate(prefix .. '.args', provider.args, { 'table', 'function' }, true)
+    vim.validate(prefix .. '.extra_args', provider.extra_args, { 'table', 'function' }, true)
     vim.validate(prefix .. '.cwd', provider.cwd, { 'string', 'function' }, true)
     vim.validate(prefix .. '.output', provider.output, { 'string', 'function' }, true)
     vim.validate(prefix .. '.error_parser', provider.error_parser, 'function', true)