fix(syntax): per-level heading groups so colorschemes differentiate H1-H6

The single `nuwikiHeading` group linked all six levels to `Title`.
Many popular colorschemes (codedark, gruvbox-light, etc.) define
`Title` as bold-only with no foreground colour, so every heading
rendered as bold text in the Normal colour — H1 and H3 were
visually identical.

Split into nuwikiHeading1..6 and link each to a different accent
(Title / Function / Identifier / Type / Constant / PreProc), matching
the per-level Tree-sitter mapping the Neovim semantic-token side
already uses. The patterns are mutually exclusive: each requires
whitespace immediately after the equals run, so `=` won't match `==`.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

syntax/vimwiki.vim

@@ -63,7 +63,15 @@ endif
 
 syntax case match
 
-syntax match nuwikiHeading      /^\s*=\{1,6}\s.\{-}\s=\{1,6}\s*$/
+" One match per heading level so colorschemes can style each differently.
+" Each pattern requires whitespace immediately after the equals run, which
+" makes the levels mutually exclusive (`==` won't match `===`).
+syntax match nuwikiHeading1      /^\s*=\s.\{-}\s=\s*$/
+syntax match nuwikiHeading2      /^\s*==\s.\{-}\s==\s*$/
+syntax match nuwikiHeading3      /^\s*===\s.\{-}\s===\s*$/
+syntax match nuwikiHeading4      /^\s*====\s.\{-}\s====\s*$/
+syntax match nuwikiHeading5      /^\s*=====\s.\{-}\s=====\s*$/
+syntax match nuwikiHeading6      /^\s*======\s.\{-}\s======\s*$/
 syntax match nuwikiHorizontal   /^-\{4,}\s*$/
 syntax match nuwikiCommentLine  /^%%.*$/
 syntax region nuwikiCommentBlock start=/^%%+/ end=/+%%/ keepend
@@ -92,7 +100,16 @@ syntax match nuwikiWikilinkTarget  /[^|\]]*|/ contained conceal
 syntax region nuwikiTransclusion start=/{{[^{]/ end=/}}/ keepend
 syntax match nuwikiUrl          /\<\(https\?\|ftp\|mailto\|file\):\/\?\/\?\S\+/
 
-highlight default link nuwikiHeading        Title
+" Heading levels map to progressively softer accents — matches the Neovim
+" semantic-token mapping above so static fallback and LSP-driven highlights
+" agree. Colorschemes that don't differentiate Title vs Function vs etc.
+" will still show all six as the same colour, but most do.
+highlight default link nuwikiHeading1       Title
+highlight default link nuwikiHeading2       Function
+highlight default link nuwikiHeading3       Identifier
+highlight default link nuwikiHeading4       Type
+highlight default link nuwikiHeading5       Constant
+highlight default link nuwikiHeading6       PreProc
 highlight default link nuwikiHorizontal     Comment
 highlight default link nuwikiCommentLine    Comment
 highlight default link nuwikiCommentBlock   Comment