fix(merge): clear resolved state on buffer re-read

Problem: resolved_hunks and virtual text persisted when a diff buffer
was re-read, showing stale (resolved) markers for hunks that were no
longer resolved.

Solution: clear resolved_hunks[bufnr] and the merge namespace at the
top of setup_keymaps so each buffer init starts fresh.

lua/diffs/merge.lua

@@ -343,6 +343,9 @@ end
 ---@param bufnr integer
 ---@param config diffs.ConflictConfig
 function M.setup_keymaps(bufnr, config)
+  resolved_hunks[bufnr] = nil
+  vim.api.nvim_buf_clear_namespace(bufnr, ns, 0, -1)
+
   local km = config.keymaps
 
   local maps = {