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>

doc/nuwiki.txt

@@ -196,6 +196,9 @@ match upstream vimwiki.
 `css_name`               `'style.css'`
 `auto_export`            `false` — export to HTML on save.
 `auto_toc`               `false` — refresh TOC on save.
+`toc_header`             `'Contents'`  (+ `toc_header_level` `1`) — :VimwikiTOC.
+`links_header`           `'Generated Links'` (+ `links_header_level` `1`).
+`tags_header`            `'Generated Tags'` (+ `tags_header_level` `1`).
 `exclude_files`          List of glob patterns excluded from export.
 `listsyms`               `' .oOX'` — checkbox progression characters.
 `listsym_rejected`       `'-'` — glyph for a cancelled checkbox `[-]`.