Problem: after closing a viewer, there was no way to re-open the last
compiled output without recompiling.
Solution: track the most recent output file per buffer in a `last_output`
table that persists after compilation finishes. Add `compiler.open()`,
`M.open()`, and wire it into the command dispatch.
Problem: :Preview watch only registered a BufWritePost autocmd without
compiling immediately, required boilerplate to open output files after
first compilation, and was misleadingly named.
Solution: Rename watch → toggle throughout. M.toggle now compiles
immediately on activation. Add an open field to ProviderConfig: true
calls vim.ui.open(), a string[] runs the command with the output path
appended, tracked per-buffer so the file opens only once. All presets
default to { 'xdg-open' }. Health check validates opener binaries.
Guard the async compile callback against invalid buffer ids.
