ww build: honor output directories

docs/spec.md

@@ -310,6 +310,35 @@ ImportPath    = ident { "." ident } .
   `fn main`; path and directory spelling do not classify commands. An ordinary
   import of a package declared `main` is rejected, except for the toolchain's
   colocated external-test wiring.
+- For `ww build`, an explicit `-o` names an output directory when ordinary
+  `stat` reports an existing directory (following symlinks) or its spelling
+  ends in `/`. This classification is independent of whether one or many
+  package roots were requested. Each selected command is published beneath
+  that directory using the final component of its requested contextual import
+  name, with the selected local directory leaf as the fallback when no such
+  identity exists. Independently selected non-main roots load but do not enter
+  the action list; non-main command dependencies retain ordinary actions, and
+  a selection containing no command rejects before tools or output creation.
+  A raw command-line `.ww` source uses its source basename without `.ww`, and
+  a raw non-main source is the same no-command rejection. Missing
+  trailing-slash hierarchies are created transactionally from `0777`, filtered
+  by the caller's umask.
+  Loading and package/import diagnostics precede no-main, derived-path,
+  duplicate-destination, implicit-default, and other output preflight; all of
+  those checks precede creation. Producer failure or interruption preserves
+  existing directory contents and removes only request-created prefixes and
+  stages. A non-directory output retains the single-product file/archive rule.
+  If a lone
+  command's synthesized default basename already names a directory, loading
+  and graph validation complete and the build rejects before tools without
+  changing that directory; a non-main package synthesizes no default public
+  output. Output paths and directory metadata never become package, import,
+  graph, action,
+  symbol, artifact, `.wwi`, or persistence identity.
+  Assembly-only `-S` retains the directory form's command-action selection and
+  no-main rejection, but it reaches no install action: destination length,
+  duplicate publication names, implicit destination collision, and output
+  parent creation are therefore inapplicable.
 - A newly published `ww build` command is created with permission `0777`
   filtered by the invoking process's umask. A newly published non-command
   archive, and the adjacent WW interface required to consume it, use `0666`