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.

lua/preview/compiler.lua

@@ -58,6 +58,14 @@ end
 function M.compile(bufnr, name, provider, ctx, opts)
   opts = opts or {}
 
+  if vim.fn.executable(provider.cmd[1]) ~= 1 then
+    vim.notify(
+      '[preview.nvim]: "' .. provider.cmd[1] .. '" is not executable (run :checkhealth preview)',
+      vim.log.levels.ERROR
+    )
+    return
+  end
+
   if vim.bo[bufnr].modified then
     vim.cmd('silent! update')
   end

spec/compiler_spec.lua

@@ -99,6 +99,35 @@ describe('compiler', function()
       helpers.delete_buffer(bufnr)
     end)
 
+    it('notifies and returns when binary is not executable', function()
+      local bufnr = helpers.create_buffer({ 'hello' }, 'text')
+      vim.api.nvim_buf_set_name(bufnr, '/tmp/preview_test_nobin.txt')
+      vim.bo[bufnr].modified = false
+
+      local notified = false
+      local orig = vim.notify
+      vim.notify = function(msg)
+        if msg:find('not executable') then
+          notified = true
+        end
+      end
+
+      local provider = { cmd = { 'totally_nonexistent_binary_xyz_preview' } }
+      local ctx = {
+        bufnr = bufnr,
+        file = '/tmp/preview_test_nobin.txt',
+        root = '/tmp',
+        ft = 'text',
+      }
+
+      compiler.compile(bufnr, 'nobin', provider, ctx)
+      vim.notify = orig
+
+      assert.is_true(notified)
+      assert.is_nil(compiler._test.active[bufnr])
+      helpers.delete_buffer(bufnr)
+    end)
+
     it('fires PreviewCompileFailed on non-zero exit', function()
       local bufnr = helpers.create_buffer({ 'hello' }, 'text')
       vim.api.nvim_buf_set_name(bufnr, '/tmp/preview_test_fail.txt')