fix(help): :helptags regressions

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.

0 files changed, 0 insertions, 0 deletions