feat(commands): add :Preview open subcommand

Problem: after closing a viewer, there was no way to re-open the last
compiled output without recompiling.

Solution: track the most recent output file per buffer in a `last_output`
table that persists after compilation finishes. Add `compiler.open()`,
`M.open()`, and wire it into the command dispatch.

lua/preview/commands.lua

@@ -1,6 +1,6 @@
 local M = {}
 
-local subcommands = { 'compile', 'stop', 'clean', 'toggle', 'status' }
+local subcommands = { 'compile', 'stop', 'clean', 'toggle', 'open', 'status' }
 
 ---@param args string
 local function dispatch(args)
@@ -14,6 +14,8 @@ local function dispatch(args)
     require('preview').clean()
   elseif subcmd == 'toggle' then
     require('preview').toggle()
+  elseif subcmd == 'open' then
+    require('preview').open()
   elseif subcmd == 'status' then
     local s = require('preview').status()
     local parts = {}
@@ -47,7 +49,7 @@ function M.setup()
     complete = function(lead)
       return complete(lead)
     end,
-    desc = 'Compile, stop, clean, toggle, or check status of document preview',
+    desc = 'Compile, stop, clean, toggle, open, or check status of document preview',
   })
 end
 

lua/preview/compiler.lua

@@ -12,6 +12,9 @@ local watching = {}
 ---@type table<integer, true>
 local opened = {}
 
+---@type table<integer, string>
+local last_output = {}
+
 ---@param val string[]|fun(ctx: preview.Context): string[]
 ---@param ctx preview.Context
 ---@return string[]
@@ -61,6 +64,10 @@ function M.compile(bufnr, name, provider, ctx)
     output_file = eval_string(provider.output, ctx)
   end
 
+  if output_file ~= '' then
+    last_output[bufnr] = output_file
+  end
+
   log.dbg('compiling buffer %d with provider "%s": %s', bufnr, name, table.concat(cmd, ' '))
 
   local obj = vim.system(
@@ -117,6 +124,7 @@ function M.compile(bufnr, name, provider, ctx)
     once = true,
     callback = function()
       M.stop(bufnr)
+      last_output[bufnr] = nil
     end,
   })
 
@@ -235,6 +243,18 @@ function M.clean(bufnr, name, provider, ctx)
   )
 end
 
+---@param bufnr integer
+---@return boolean
+function M.open(bufnr)
+  local output = last_output[bufnr]
+  if not output then
+    log.dbg('no last output file for buffer %d', bufnr)
+    return false
+  end
+  vim.ui.open(output)
+  return true
+end
+
 ---@param bufnr integer
 ---@return preview.Status
 function M.status(bufnr)
@@ -254,6 +274,7 @@ M._test = {
   active = active,
   watching = watching,
   opened = opened,
+  last_output = last_output,
 }
 
 return M

lua/preview/init.lua

@@ -39,6 +39,7 @@
 ---@field stop fun(bufnr?: integer)
 ---@field clean fun(bufnr?: integer)
 ---@field toggle fun(bufnr?: integer)
+---@field open fun(bufnr?: integer)
 ---@field status fun(bufnr?: integer): preview.Status
 ---@field get_config fun(): preview.Config
 local M = {}
@@ -166,6 +167,14 @@ function M.toggle(bufnr)
   compiler.toggle(bufnr, name, provider, M.build_context)
 end
 
+---@param bufnr? integer
+function M.open(bufnr)
+  bufnr = bufnr or vim.api.nvim_get_current_buf()
+  if not compiler.open(bufnr) then
+    vim.notify('[preview.nvim] no output file available for this buffer', vim.log.levels.WARN)
+  end
+end
+
 ---@class preview.Status
 ---@field compiling boolean
 ---@field watching boolean