fix(lua): recognize spaceless headings (==Heading==) in client helpers

The shared heading_level helper still required spaces around the title, so
the client-side regex paths — fold fallback, text objects (ah/ih), and
heading navigation (]] etc.) — skipped spaceless headings, even though the
server (and HTML export) accept them since the #7 fix. Rewrite to match the
lexer: balanced leading/trailing `=` runs (cap 6) around a non-empty title,
spaced or spaceless. Marker-only (`======`), unbalanced (`==a==b`, `==> x`)
and 7+ runs still return 0.

Verified against an 18-case battery + full editor harness (10/10).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lua/nuwiki/util.lua

@@ -6,9 +6,14 @@ local M = {}
 
 -- Heading level of `line` (1-6), or 0 when it isn't a heading. Mirrors the
 -- server lexer: balanced leading/trailing runs of `=` around non-empty text,
--- capped at level 6 (a 7+ `=` run is not a heading). `nil`-safe.
+-- capped at level 6 (a 7+ `=` run is not a heading). Both the spaced
+-- (`== H ==`) and spaceless (`==H==`) forms are accepted, matching vimwiki.
+-- `nil`-safe.
 function M.heading_level(line)
-  local lead = line and line:match('^%s*(=+)%s')
+  if not line then
+    return 0
+  end
+  local lead = line:match('^%s*(=+)')
   if not lead then
     return 0
   end
@@ -16,10 +21,17 @@ function M.heading_level(line)
   if lvl > 6 then
     return 0
   end
-  local trail = line:match('%s(=+)%s*$')
+  -- Same-length trailing run of `=` at end of line (after optional trailing ws).
+  local trail = line:match('(=+)%s*$')
   if not trail or #trail ~= lvl then
     return 0
   end
+  -- Non-empty title between the two runs (rules out marker-only lines like
+  -- `======`). `(.-)` is lazy, so the leading/trailing `=+` keep their runs.
+  local body = line:match('^%s*=+(.-)=+%s*$')
+  if not body or body:match('^%s*$') then
+    return 0
+  end
   return lvl
 end