fix(vim): follow-link creates missing pages + safe LSP foldexpr

Two user-visible bugs:

1. `<CR>` on a wikilink whose target wasn't indexed yet (typical for
   "follow this link → page doesn't exist → I want a fresh buffer for
   it") did nothing. Cause: `Backend::resolve_target_uri` returned
   `None` once `WorkspaceIndex::resolve` came up empty, so
   `goto_definition` had no Location to hand back to the client and
   `vim.lsp.buf.definition()` / `:LspDefinition` silently no-op'd.

   Vimwiki's behaviour is "open a fresh buffer for the future page".
   `Backend::resolve_target_uri` now synthesises a
   `<wiki_root>/<path><file_extension>` URI when the lookup misses,
   for both `LinkKind::Wiki` and the `LinkKind::Interwiki` fallback.
   The editor opens an empty buffer; saving writes the file. New
   helper `synthesise_page_uri` does the path walk so subdirectory
   wikilinks like `[[notes/Daily]]` get the right output path.

2. Folding occasionally surfaced an `E5108` from inside
   `vim.lsp.foldexpr()` — happens when the function fires before the
   LSP client has finished attaching to the buffer, or when the
   server's `foldingRange` response hasn't yet arrived. The error
   gets shouted once per visible line.

   `lua/nuwiki/folding.lua` now exports `lsp_expr` which `pcall`s
   `vim.lsp.foldexpr` and falls back to the regex implementation
   (`M.expr`) on error or when the LSP function isn't available.
   `ftplugin.lua` points `foldexpr` at the wrapper instead of
   `vim.lsp.foldexpr` directly. `foldtext` is set in both branches
   now so collapsed folds keep the nicer summary line.

Tests: 4 new in `phase19_followlink_creates.rs` covering indexed
resolution, missing-page fallback, the canonical
`<root>/<page>.wiki` shape, and the slash-bearing subdirectory link.
Total 381 tests pass; fmt + clippy clean.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lua/nuwiki/folding.lua

@@ -39,4 +39,20 @@ function M.foldtext()
   return string.format('▸ %s … (%d lines)', line, count)
 end
 
+--- LSP-backed foldexpr with a safe fallback.
+---
+--- Wraps `vim.lsp.foldexpr` so a missing client / un-attached buffer /
+--- transient evaluation error falls back to the regex implementation
+--- instead of throwing E5108 for every line in the buffer.
+function M.lsp_expr()
+  if not (vim.lsp and vim.lsp.foldexpr) then
+    return M.expr()
+  end
+  local ok, val = pcall(vim.lsp.foldexpr)
+  if ok then
+    return val
+  end
+  return M.expr()
+end
+
 return M

lua/nuwiki/ftplugin.lua

@@ -13,6 +13,10 @@ local function setup_folding(bufnr, folding_mode)
   -- The ftplugin runs during BufRead → FileType, by which time the
   -- buffer is displayed in the current window. `vim.opt_local` writes
   -- to the current window's view of the option.
+  -- The LSP foldexpr is wrapped via `nuwiki.folding.lsp_expr` so a
+  -- transient client failure (e.g. server still starting, buffer not
+  -- yet attached) degrades to the regex variant instead of spamming
+  -- the user with E5108 errors per line.
   local apply = function()
     if folding_mode == 'expr'
        or not (vim.fn.has('nvim-0.11') == 1 and vim.lsp.foldexpr)
@@ -21,9 +25,9 @@ local function setup_folding(bufnr, folding_mode)
       vim.opt_local.foldexpr = 'v:lua.require("nuwiki.folding").expr()'
       vim.opt_local.foldtext = 'v:lua.require("nuwiki.folding").foldtext()'
     else
-      -- LSP-backed folding via Neovim 0.11+'s built-in foldexpr.
       vim.opt_local.foldmethod = 'expr'
-      vim.opt_local.foldexpr = 'v:lua.vim.lsp.foldexpr()'
+      vim.opt_local.foldexpr = 'v:lua.require("nuwiki.folding").lsp_expr()'
+      vim.opt_local.foldtext = 'v:lua.require("nuwiki.folding").foldtext()'
     end
   end