Problem: Argument type mismatches are excluded from EmmyLua checks.

Solution: Correct runtime annotations and calls, document local checker
workarounds, and enable the diagnostic.

AI-assisted

Enable EmmyLua's need-check-nil diagnostic and resolve nullable values
through more precise types, explicit invariants, and guards for missing
runtime state. Keep analyzer-specific workarounds local and update the
affected API documentation.

Exclude the bundled re.lua from analysis so its nil-check warnings do
not fail CI.

AI-assisted

Make gen_helptags.lua call help.lua instead of keeping its own tag
parser. Builds and :helptags now use the same code.

Allow the shared code to run without Neovim, and fail the build when tag
generation reports an error.

AI-assisted

Problem: Generating helptags builds and queries a full vimdoc syntax
tree. This is much slower than the previous scanner and also happens
before discovering that the tags file cannot be written.

Solution: Extract tags with an LPeg grammar, reading each file once and
skipping example blocks and ordinary text. Open the output first so
unwritable directories can return immediately.

This restores legacy handling of tags inside inline code spans. On an
arm64 Mac, the 5.36 MB runtime corpus takes 24 ms instead of 604 ms,
with identical generated tags.

AI-assisted

Enable checks for unused code, deprecated calls, return types, and
annotations.

Fix the warnings and replace the deprecated highlight function. Keep
exceptions where old APIs are still needed or the checker gets it wrong.

AI-assisted

Problem: K on generic and nullable type annotations can jump to the
wrong help tag. Generated See references repeat the type names.

Solution: Strip generic arguments and nullable suffixes when resolving
help tags. Remove the redundant generated See references.

AI-assisted

Problem:
The vimdoc parser does not treat "\r" as whitespace, so in a CRLF
helpfile the codeblock rule fails and "*" pairs inside examples become
tags. On Windows this generates duplicate "." and "/" tags:

    D:/a/neovim/neovim/build/bin/nvim.exe -u NONE -i NONE -e --headless -c "helptags ++t doc" -c "exe 'cquit' !empty(v:errmsg)""
    Error in command line:
    E154: Duplicate tag "." in gui.txt and repeat.txt
    E154: Duplicate tag "/" in pattern.txt and usr_08.txt
    E154: Duplicate tag "/" in usr_08.txt and pattern.txt

Note: The old C parser was unaffected because it read helpfiles in text
mode (`os_fopen(…, "r")`).

Solution:
Strip "\r" before parsing.

Problem:
Parent commit regressed some behavior of the old C helptags-gen impl:
- helpfiles in sub-directories are named by basename, so :help fails
- "help-tags" always names "tags", never "tags-nl"
- E150 E151 E152 E153 are never reported
- existing tags file is not overwritten if no tags were found
- a duplicate tag aborts the run, skipping the remaining directories
- `*.TXT` and `*.FRX` (uppercase) are not recognized as helpfiles
- tree-sitter-vimdoc accepts tags that the C parser rejected: `*a|b*`
  (breaks |links|) and unterminated `*tag`
- PUC Lua "<" compares with (localized) `strcoll()`, so the tags file is
  not sorted by byte value (E432)
- requiring `vim.treesitter` at load time breaks :help itself, not just
  :helptags, where the module is unavailable
- `vim.pack` runs :helptags for plugins that have no "doc/" directory,
  so every install/update emits E150

Solution:
- Fail the build if generating helptags reports any `v:errmsg`.
- Restore old behavior: tags generated for runtime/doc are now identical
  to those from the C implementation. Errors are non-fatal messages
  instead of exceptions, so all directories are still processed.
- Load treesitter lazily, report a plain error if parser is missing.

Problem:
Tags are manually parsed in C which is not flexible and prone to errors.
Extending the help system to allow for other formats (e.g. Markdown)
would require a large rewrite in the C core, while with Treesitter it
only needs a query update.

Solution:
Use the power of treesitter to extract the tags from helpfiles.

- build: set `$VIMRUNTIME` when generating helptags, like
  `cmake/Util.cmake` already does for other generators.
- fix(help): only accept tags delimited by whitespace. The old C parser
  only accepted a `*tag*` preceded by start-of-line or whitespace and
  followed by whitespace or end-of-line. The vimdoc parser also captures
  tags followed by other text, e.g. `*$XDG_STATE_HOME*/.../logs` in
  starting.txt, which caused an E154 duplicate tag error for docs that
  were previously fine.