fix: use non-NUL separator in mermaid_cache_path

The NUL byte separator in mermaid_cache_path causes the Lua→vimscript
bridge to convert the concatenated string to a Blob on Neovim ≤ 0.11,
which vim.fn.sha256 rejects with E976. Using '|' avoids the Blob
conversion entirely. Fixed upstream in v0.12 (vim-patch 9.1.1774).

Author: hambergerpls

lua/md-render/image.lua

@@ -407,7 +407,7 @@ end
 ---@return string
 local function mermaid_cache_path(source)
   local theme, bg_hex = mermaid_theme_args()
-  local hash = vim.fn.sha256(source .. "\0" .. theme .. "\0" .. bg_hex):sub(1, 16)
+  local hash = vim.fn.sha256(source .. "|" .. theme .. "|" .. bg_hex):sub(1, 16)
   return get_mermaid_cache_dir() .. "/" .. hash .. ".png"
 end