* refactor: rename build to compile and watch to toggle in public API
Problem: the code used build/watch while the help file already
documented compile/toggle, creating a confusing mismatch.
Solution: rename M.build() to M.compile() and M.watch() to M.toggle()
in init.lua, update handler keys in commands.lua, and update the test
file to match.
* refactor(commands): make toggle the default subcommand
Problem: bare :Preview ran a one-shot compile, but users reaching for a
"preview" plugin expect it to start previewing (i.e. watch mode).
Solution: change the fallback subcommand from compile to toggle so
:Preview starts/stops auto-compile on save.
* refactor(commands): remove stop subcommand
Problem: :Preview stop had a subtle distinction from toggle-off (kill
process but keep autocmd) that nobody reaches for deliberately from
the command line.
Solution: remove stop from the command dispatch table. The Lua API
require('preview').stop() remains as a programmatic escape hatch.
* docs: update help file for new command surface and document reload
Problem: the help file listed compile as the default subcommand, still
included the stop subcommand, omitted the reload provider field, and
had a misleading claim about shipping with zero defaults.
Solution: make toggle the default in the commands section, remove stop
from subcommands, add reload to provider fields, fix the introduction
text, reorder API entries to match new primacy, and add an output path
override example addressing #26/#27.
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