Improve the vim.iter annotations with richer generics that track element and
tuple types through iterator pipelines, including multi-value stages and
list-specific methods.
Extend the LuaCATS parser and vimdoc generator so those richer generic classes
and overloads round-trip into the generated help. These annotations are only
supported by EmmyLua, so LuaLS still uses a broader fallback in _meta.lua.
AI-assisted: Codex
d7ef55e881
