Problem: filetype detection does not run on piped input
Solution: add `StdinReadPost` to main filetype.lua autocommand
Rationale: legacy filetype detection checked contents by sourcing
`scripts.vim` in separate autocommands, including on `StdinReadPost`.
For Lua filetype detection, this was moved into the main autocommand,
with bundled `scripts.vim` gated behind `g:do_legacy_filetype` (i.e.,
only user `scripts.vim` are sourced for compatibility by default). Adding
`StdinReadPost` to the main autocommand again runs content checks on
piped input without requiring code duplication and low-payoff
refactoring.