ci(keymaps): pin Neovim 0.11 from upstream release tarball

CI log for the keymaps job showed Lua crashing with
`attempt to call field 'get_clients' (a nil value)` plus a flood of
`W18: Invalid character in group name` on the `@vimwiki*` highlight
links. Both signal that apt's `neovim` package is too old (Ubuntu
LTS ships 0.6/0.7) — `vim.lsp.get_clients` only landed in 0.10, and
Tree-sitter-style `@group.modifier` highlight names only became
valid sometime in the 0.8-ish era.

Switched the installer to pull the official static tarball
(`neovim/neovim/releases/v0.11.3/nvim-linux-x86_64.tar.gz`) and
symlink it into `/usr/local/bin/nvim`. Vim from apt is kept — it
only runs the pure-VimL harness so its age doesn't matter.

Also added a defensive fallback in the harness:
`get_lsp_clients = vim.lsp.get_clients or vim.lsp.get_active_clients`,
plus a `server_capabilities` nil guard. Won't matter on the pinned
0.11 runner, but means a future regression that drops the deps will
fail with a clear `lsp.attached` FAIL message instead of a Lua
stack trace.

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

scripts/test-keymaps.lua

@@ -82,11 +82,18 @@ local function run(name, opts)
   record(ok, name, ok and '' or tostring(err))
 end
 
+local function get_lsp_clients(opts)
+  local fn = vim.lsp.get_clients or vim.lsp.get_active_clients
+  if not fn then return {} end
+  return fn(opts)
+end
+
 local function wait_for_lsp(timeout_ms)
   local deadline = vim.loop.now() + (timeout_ms or 5000)
   while vim.loop.now() < deadline do
-    local clients = vim.lsp.get_clients({ name = 'nuwiki', bufnr = 0 })
-    if #clients > 0 and clients[1].server_capabilities.executeCommandProvider then
+    local clients = get_lsp_clients({ name = 'nuwiki', bufnr = 0 })
+    if #clients > 0 and clients[1].server_capabilities
+        and clients[1].server_capabilities.executeCommandProvider then
       return clients[1]
     end
     sleep(100)