fix: lifecycle cleanup and defensive runtime checks (#29)

* fix(commands): register VimLeavePre to call stop_all

Problem: spawned compiler processes and watching autocmds were never
cleaned up when Neovim exited, leaving orphaned processes running.

Solution: register a VimLeavePre autocmd in commands setup that calls
compiler.stop_all(), which kills active processes, unwatches all
buffers, and stops the reload server.

* fix(compiler): replace BufWipeout with BufUnload

Problem: cleanup autocmds used BufWipeout, which only fires for
:bwipeout. The common :bdelete path (used by most buffer managers
and nvim_buf_delete) fires BufUnload but not BufWipeout, so processes
and watches leaked on normal buffer deletion.

Solution: switch all three cleanup autocmds from BufWipeout to
BufUnload, which fires for both :bdelete and :bwipeout.

* fix(init): guard against unnamed buffer in public API

Problem: calling compile/toggle/clean/open on an unsaved scratch
buffer passed an empty string as ctx.file, producing nonsensical
output paths like ".pdf" and silently passing empty strings to
compiler binaries.

Solution: add an early return with a WARN notification in compile,
toggle, clean, and open when the buffer has no file name.

* fix(compiler): add fs_stat check to one-shot open path

Problem: the long-running process path already guarded opens with
vim.uv.fs_stat(), but the one-shot compile path and M.open() did not.
Compilation can exit 0 and produce no output, and output files can be
externally deleted between compile and open.

Solution: add the same fs_stat guard to the one-shot open branch and
to M.open() before attempting to launch the viewer.

* fix(compiler): check executable before spawning process

Problem: if a configured binary was missing or not in PATH, vim.system
would fail silently or with a cryptic OS error. The user had no
actionable feedback without running :checkhealth.

Solution: check vim.fn.executable() at the start of M.compile() and
notify with an ERROR-level message pointing to :checkhealth preview
if the binary is not found.

* fix(compiler): reformat one-shot open condition for line length

Problem: the added fs_stat condition exceeded stylua's line length
limit on the one-shot open guard.

Solution: split the boolean condition across multiple lines to match
the project's stylua formatting rules.

spec/init_spec.lua

@@ -108,4 +108,62 @@ describe('preview', function()
       helpers.delete_buffer(bufnr)
     end)
   end)
+
+  describe('unnamed buffer guard', function()
+    before_each(function()
+      helpers.reset_config({ typst = true })
+      preview = require('preview')
+    end)
+
+    local function capture_notify(fn)
+      local msg = nil
+      local orig = vim.notify
+      vim.notify = function(m)
+        msg = m
+      end
+      fn()
+      vim.notify = orig
+      return msg
+    end
+
+    it('compile warns on unnamed buffer', function()
+      local bufnr = helpers.create_buffer({}, 'typst')
+      local msg = capture_notify(function()
+        preview.compile(bufnr)
+      end)
+      assert.is_not_nil(msg)
+      assert.is_truthy(msg:find('no file name'))
+      helpers.delete_buffer(bufnr)
+    end)
+
+    it('toggle warns on unnamed buffer', function()
+      local bufnr = helpers.create_buffer({}, 'typst')
+      local msg = capture_notify(function()
+        preview.toggle(bufnr)
+      end)
+      assert.is_not_nil(msg)
+      assert.is_truthy(msg:find('no file name'))
+      helpers.delete_buffer(bufnr)
+    end)
+
+    it('clean warns on unnamed buffer', function()
+      local bufnr = helpers.create_buffer({}, 'typst')
+      local msg = capture_notify(function()
+        preview.clean(bufnr)
+      end)
+      assert.is_not_nil(msg)
+      assert.is_truthy(msg:find('no file name'))
+      helpers.delete_buffer(bufnr)
+    end)
+
+    it('open warns on unnamed buffer', function()
+      local bufnr = helpers.create_buffer({}, 'typst')
+      local msg = capture_notify(function()
+        preview.open(bufnr)
+      end)
+      assert.is_not_nil(msg)
+      assert.is_truthy(msg:find('no file name'))
+      helpers.delete_buffer(bufnr)
+    end)
+  end)
 end)