parse: enforce strict-package — reject package-less files (#24a)

Flip the soft-default to a hard "missing package clause" error symmetrically in
both stages (cmd/wcc/parse.c + lib/ww/syntax/parse.ww): the first real decl of a
primary section with empty pathmod/resetmod and no seen clause is now rejected.
Closes the documented soft-default divergence (the 63-wrapper carve-out).

The gate flip can't be split from the migration it breaks, so this is one atomic
commit: ~80 test/wcc wrappers gain `package main;` via a shared wwtestpkg.h
helper, 6 data fixtures plus 17 asm-grep assertions update for the bare->main.<leaf>
root-helper mangle shift, and rt/ declares `package rt;` with @symbol pinning the
bare rt_ensure/rt_malloc linker names.

Root mangling narrows: the executable entry `main` stays bare (existing
carve-out), but root helper symbols become main.X. The #84 cluster is rewritten
to assert main.run distinct from aa.run/test.run; its cgen fix and bare machinery
are retained — still load-bearing for package-less module-reset deps. New
table-driven test 782_strict_package.c (6 rows, both stages).

Retiring //ww:module-reset is deferred to #24b: it is load-bearing (clears the
.wwi pathmod so the body's package clause asserts), not a vestige; fusing its
removal here would be a silent mismatch.

All byte-id gates green; full make test reports "all 335 tests passed".

test/wcc/724_letdecl_zeroinit.c

@@ -29,6 +29,7 @@
 #include <unistd.h>
 #include <sys/stat.h>
 #include <sys/wait.h>
+#include "wwtestpkg.h"
 
 static int
 runwait(const char *cmd)
@@ -79,7 +80,7 @@ check_movq_zero_present(const char *spath, const struct row *r)
 	char buf[1 << 16];
 	if (slurp(spath, buf, sizeof buf) < 0) return -1;
 
-	const char *body = strstr(buf, "TEXT produce");
+	const char *body = strstr(buf, "TEXT main.produce");
 	if (!body) {
 		fprintf(stderr, "row[%s]: no TEXT produce label\n", r->label);
 		return -1;
@@ -112,7 +113,7 @@ emit_s(const char *w6c, const struct row *r, int i, char *out_s, size_t cap)
 
 	FILE *f = fopen(src, "wb");
 	if (!f) return -1;
-	fputs(r->src, f);
+	wwtest_fputs(r->src, f);
 	fclose(f);
 
 	snprintf(cmd, sizeof cmd, "%s -o %s %s 2>/dev/null", w6c, out_s, src);