ww build: discard exact null outputs

docs/spec.md

@@ -319,6 +319,19 @@ ImportPath    = ident { "." ident } .
   build therefore reuses unchanged semantic actions while refreshing the
   caller-visible output with the current invocation's mode. Assembly-only
   builds publish no executable or archive.
+- On the Unix target, exact `-o /dev/null` is a build-output discard request,
+  not an ordinary output filename. Package loading, import validation, graph
+  construction, compilation, assembly, archiving, command linking, failure,
+  and persistent invalidation proceed normally; only installation of a
+  caller-visible command, archive, interface, default basename, or adjacent
+  scratch tree is omitted. One such request may select any number of command
+  and non-command packages, and an empty recursive match is successful after
+  its normal warning. `-S` likewise needs no caller workdir merely to retain
+  discarded assembly. Raw-file and directory roots use the same rule. A
+  non-exact spelling is an ordinary output and keeps all established fan-out,
+  rejection, permission, scratch, and publication behavior. Output discard is
+  request metadata; it does not change package, import, graph, action,
+  artifact, symbol, `.wwi`, or persistence identity.
 - Only names marked `export` (§5) are visible across module boundaries.
 
 Import paths remain unquoted and dotted. Grouped imports, quoted import paths,
@@ -662,7 +675,11 @@ directory. An output ending in `/` or naming an existing directory receives
 that basename and may have missing parent directories created. One
 non-directory output may name only one package. Multiple packages whose
 visible import leaves would produce the same test-binary name reject before
-tools or output creation; exact `/dev/null` is the discard exception. Declared
+tools or output creation; exact `/dev/null` is the discard exception. It keeps
+the ordinary private link and, unless `-c` is present, the ordinary execution,
+but installs no retained copy. The raw single-file compatibility route uses
+the same private-output rule and never treats `/dev/null` as an explicit
+artifact or adjacent-scratch stem. Declared
 package names, test variants, source filenames, physical directories, output
 paths, and retained binary names remain presentation or loader metadata and do
 not become canonical package or action identity.