fix: resolve internal links by target instead of link text

[geodimm]

Writer.Inline.Link built the help reference for a [text](#anchor) link by slugifying the link's text, ignoring the target entirely. A link only resolved when its text happened to spell the target heading, so any other wording produced a |reference| matching no tag, and :help on it failed with E149.

The failure was silent: pandoc exited 0, :helptags exited 0, and the README rendered correctly on GitHub.

Writer.Pandoc now walks the document before rendering and maps each heading's identifier to the tag that heading will emit, including the parent prefix added under --dedup-subheadings. Links resolve against that map, so the reference follows the target.

Link text that adds something is kept ahead of the tag, matching how external links already render as text <target>:

See [Options](#options) and [the options below](#options).

See |project-options| and the options below |project-options|.

Text that merely repeats the heading is dropped, since the tag already spells it out, and the comparison ignores case and markup so Options is treated as a repeat.

Headings below level 2 are rendered without a tag, so links to them cannot resolve. Those now render as plain text with a warning on stderr rather than emitting a reference that cannot be followed.

Verified against two published plugin READMEs: output is byte-identical for one whose link text already matched its headings, and the other's eleven dangling references drop to zero.

Fixes #75

[kdheepak]

Thanks for the PR with tests!

[geodimm]

Thank you for the quick turnaround. Are there any plans to create a new tag and/or release or should I switch to main? I am asking because I'd like to use the Nix package but it's quite old: https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/pa/panvimdoc/package.nix

[kdheepak]

Thanks for the ping, I made a new release! https://github.com/kdheepak/panvimdoc/releases/tag/v5.0.0