Remove deferred compilation and $compile(); add standalone file operations

[jgabry]

Remove deferred compilation and $compile(); add standalone file operations

Design note: #1254.

cmdstan_model(compile = FALSE) and the $compile() method both go. Options are one-shot: a build is configured on the call that performs it (#1254 §2). The replacement API (compile_stan_file(), format_stan_file(), check_syntax_stan_file(), stan_variables(), stan_build_info() and the shared compile_impl()) is #1254 §8 ("Contract: the API that replaces deferred compilation"), including the names, the argument lists and why $compile() is removed rather than narrowed.

Stage 4 (#1258), shipping together with #1255: if $compile() goes before configuration mismatches trigger rebuilds, an existing unthreaded executable is still reused under today's logic while the only escape route is gone.

Current usage to migrate: 96 compile = FALSE in tests, 10 in R/, 9 in man/, 5 in vignettes.

Checklist

• compile_impl() lifted from today's $compile(), returning path, record, src_info and hpp_code; compile_stan_file() and cmdstan_model() are its only callers (Design note: v1.0 compilation state and C++ options #1254 §8, "compile_stan_file() is exported, and shares one implementation")
• The standalone family exported, every one that hands a source file to stanc taking include_paths; _pkgdown.yml entries added and $compile()'s removed
• compile_model_methods and compile_standalone removed (Design note: v1.0 compilation state and C++ options #1254 §8, "compile_model_methods and compile_standalone are removed")
• dry_run internal only; its 22 test uses are what make migrating the compile = FALSE sites affordable
• Remove precompile_stanc_options_, precompile_include_paths_, precompile_cpp_options_, the %||% include-path baseline and include_paths_dirty_ (R/model.R:642-656)
• $format() stops refreshing stan_code_ and variables_ (R/model.R:1309-1311); reformatting in place now costs a recompile, with a NEWS entry (Design note: v1.0 compilation state and C++ options #1254 §5, "Introspection is a construction-time snapshot")
• Closes $check_syntax() and $format() inherit stanc options meant for compilation #1252: its premise, compile-time stanc options leaking into $check_syntax() and $format(), disappears with the precompile store
• stan_variables() is the one name to revisit before it ships: metadata()$stan_variables on fit objects (R/csv.R:362) means the output variables, not the program's declarations