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

2 spaces is more visually distinct at very little cost.

Problem: Health check floating window gets closed when pressing 'gO' to show TOC because LSP floating preview system auto-closes on BufEnter events triggered by :lopen.

Solution: Temporarily disable BufEnter event for the current window during TOC operations and adjust window layout to prevent overlap.

Problem:
The check for concealing paths in TOCs in the qf syntax file fails
because the TOC tile has changed.

Solution:
Force the qf syntax file to be reloaded after the qf_toc variable
has been set, so that the it can apply the correct settings.

Using the explicit qf_toc key, already used in the syntax file, instead
of the title is less prone to breaking.

It was also already being set for man pages but it had no effect because
the syntax file had already been loaded when the variable was set.

Fixes #33733

Problem: repeated gO in markdown etc. adds extra toc indentation

Solution: don't memoize heading table which gets modified

Problem:
gO always says "Help TOC".

Solution:
Use a generic title instead.