ww source: align UTF-8 BOM placement

docs/test-system-v2.md

@@ -415,6 +415,20 @@ generation. The directory command retains its existing attributable final
 diagnostic. This ordering state is per source and never package, import,
 variant, action, artifact, publication, or persistence identity.
 
+Every eligible source also owns an independent source-start position. An exact
+UTF-8 BOM (`EF BB BF`) at byte offset zero is ignored before package-clause and
+import parsing, so production, same-package, external-package, and test-only
+sources all count the marker's three bytes and begin the following token at
+logical line 1 column 4. The same code point anywhere else,
+including a comment or literal, is a source error before variant actions,
+generated main, producers, test runtime, accounting, or retained publication.
+The shared coordinator applies the offset-zero rule while classifying package
+clauses; the selected stage driver scans the complete source and preserves the
+per-file rule while composing package units. Wrong-target files remain excluded
+before either operation. The marker is source representation only and never
+package/import/action/artifact/publication/persistence identity; no test result
+is cached.
+
 After that eligibility boundary and the coordinator's required package-clause
 classification and production `@test` validation parses, the delegated loader
 performs selected-basename Go 1.26.5 simple-fold preflight before its graph