ww build: honor output directories

docs/test-system-v2.md

@@ -234,6 +234,28 @@ package with no selected test source validates production
 but publishes nothing and does not create an otherwise unneeded output
 directory. Successful `-c` output is silent apart from no-test reporting.
 
+The package suite also pins the adjacent `ww build` output-directory branch.
+After normal loading, an existing directory (including a symlink to one) or an
+explicit spelling ending in `/` receives each command under its requested
+import-leaf name (or local directory-leaf fallback) even for exactly one
+selected root; a raw `.ww` command uses its source basename. A missing
+trailing-slash hierarchy is created from `0777`, filtered by umask, by the
+request transaction. A directory or raw no-main-only selection and an import
+failure run no tools and do not create output. Independently selected non-main
+siblings in a mixed request load but have no action; command dependencies still
+build. Package/import rejection precedes no-main, derived destination length,
+duplicate command basename, and implicit default-directory preflight. If the
+synthesized default command basename already exists as a directory,
+load/graph validation precedes a zero-tool rejection and preserves that
+directory; a non-main package has no corresponding default output.
+Persistent work reuse, invalidation, failures, interruption, and
+concurrent drivers retain the same action and rollback rules; output form does
+not enter package/import/action identity. This build branch starts no runtime
+process, while direct execution of its published binary remains an artifact
+check rather than part of `ww build`. Assembly-only `-S` still selects command
+actions and rejects a no-command directory request, but it performs no
+install-only destination validation or output-directory creation.
+
 The retained executable is byte-identical to the temporary runnable and has
 executable mode `0777` filtered by the caller's umask, but it is never the path
 executed by the coordinator. Publication participates in the driver's one
@@ -312,6 +334,24 @@ precede those coordinator-owned source-validation parses.
 Separate compilation is the only driver build path; no compatibility mode
 switch remains.
 
+`test/package/package_test.ww` owns the focused dual-stage
+`single_root_build_output_directory` observer. It covers existing and missing
+directory spellings; trailing and non-trailing forms; literal, logical,
+default-dot, symlink, raw-file, and implicit-default routes; build/test
+adjacency; mixed and repeated roots; command-only publication, no-main and
+implicit-collision rejection, non-main default behavior; import
+diagnostic precedence over no-main, derived-path, duplicate-destination, and
+recursive implicit-collision checks; `-S -w` command selection with long,
+duplicate, implicit, and missing-directory publication preflight omitted;
+cold, warm, and invalidated persistence;
+compiler, assembler, and linker action traces; compiler/linker failure and
+linker signal; prior-state and directory-creation rollback; concurrent
+isolation; executable modes and runtime exit values; `.new`/transaction
+cleanup; and Cstage/WWstage diagnostic, binary, assembly, and persistent
+semantic-artifact byte parity. General output permissions, occupied stages,
+multi-product transactions, null discard, and test runtime timeout remain with
+their existing observers.
+
 `test/package/package_test.ww` also owns the dual-stage
 `declared_name_identity_and_file_import_scope` and
 `explicit_import_alias_binding_modes` observers. They generate temporary