feat(config): configurable generated-section captions (P2)

TOC / links / tags section headings (and their levels) were hardcoded.
Added per-wiki toc_header/toc_header_level, links_header/links_header_level,
tags_header/tags_header_level — defaults Contents / Generated Links /
Generated Tags at level 1, matching upstream (also fixes the tags index
heading: Tags -> Generated Tags).

A caption_line(name, level) helper emits the `=`-markers; the configured
text + level thread through toc_edit / links_edit / tag_links_edit and the
auto_toc save hook. find_section_range matches the configured text, so
regeneration stays idempotent at any level.

Tests: captions_honour_custom_header_and_level (link_health) + config
round-trip + default assertions (index_and_config); existing tag/toc/links
tests updated for the new arity and the Generated Tags default. fmt/clippy
clean (8-arg ops get allow(too_many_arguments), matching the codebase
precedent); 0 Rust test failures; config-parity green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

README.md

@@ -231,6 +231,9 @@ require('nuwiki').setup({
       css_name         = 'style.css',
       auto_export      = false,          -- export on save
       auto_toc         = false,          -- auto-refresh TOC on save
+      toc_header       = 'Contents',     -- :VimwikiTOC caption (+ toc_header_level = 1)
+      links_header     = 'Generated Links',
+      tags_header      = 'Generated Tags',
       exclude_files    = {},             -- glob patterns
       -- Checkbox progression characters: " .oOX" by default.
       listsyms         = ' .oOX',
@@ -333,6 +336,9 @@ Each is a boolean. All default to `true` except `mouse`.
 | `list_margin` | int | `-1` | `-1` (auto) or `≥ 0` |
 | `links_space_char` | string | `' '` | char that replaces spaces in synthesised link paths |
 | `auto_toc` | bool | `false` | `true` \| `false` |
+| `toc_header` / `toc_header_level` | string / int | `'Contents'` / `1` | `:VimwikiTOC` caption + heading level |
+| `links_header` / `links_header_level` | string / int | `'Generated Links'` / `1` | `:VimwikiGenerateLinks` caption + level |
+| `tags_header` / `tags_header_level` | string / int | `'Generated Tags'` / `1` | `:VimwikiGenerateTagLinks` index caption + level |
 
 #### HTML export keys (per-wiki)