wcc/ww: tag sep-built dotted-path packages by full path not leaf (#57)

A separately-compiled package's primary body was emitted under a bare
`//ww:module-reset`, so its own `package <leaf>;` clause set curmod to
the leaf (e.g. utf8) while the importer spliced the .wwi under the full
`//ww:module encoding.utf8` — definer mangled `utf8.X`, importer wanted
`encoding.utf8.X`, unresolved. Thread the dotted path through the
directive: `//ww:module-reset <path>` sets curmod to the dotted path
(imported stays 0, so the root `fn main` stays bare per #32), and the
body's package clause is demoted to a leaf==last-component assertion
instead of overwriting curmod. Aligns sep-build to the M1 path-mangle
model; only the SEP emitter changes (the combined build_one arm is
untouched, so all combined byte-id gates hold). Both stages mirrored.

Commit-6 broad-soak prerequisite. Gate 989_sepdotpath_run sep-builds a
2-level dotted package and proves definer==importer qualification +
single-component non-vacuity, cs==ww.

cmd/wcc/ww.h

@@ -221,6 +221,10 @@ struct Lex {
 	const char *modpath;	/* a `//ww:module <path>` directive was seen in
 				 * the last skipped run; lexnext emits TK_MODPATH
 				 * carrying this dotted path (M1 #22). */
+	const char *modresetpath;	/* a `//ww:module-reset <path>` directive
+				 * was seen; the next TK_MODRESET carries this
+				 * dotted path so the sep primary body mangles
+				 * on the path, not its leaf clause (#57). */
 };
 
 void	lexinit(Lex*, Arena*, const char *file, const char *src, u64 len);
@@ -379,6 +383,12 @@ struct Parser {
 				 * import path; while set, decls stamp
 				 * module=pathmod and imported=1, and the
 				 * in-file `package` clause is an assertion. */
+	const char *resetmod;	/* #57: active `//ww:module-reset <path>` dotted
+				 * path; mangles decls on the path WITHOUT
+				 * imported=1 (primary-ness for -c and the #32
+				 * bare-main rule stay intact), and the in-file
+				 * `package` clause asserts (leaf == last
+				 * component) instead of overwriting curmod. */
 };
 
 void	parserinit(Parser*, Arena*, Lex*);