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/400_w6c.c

@@ -8,6 +8,7 @@
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
+#include "wwtestpkg.h"
 
 static int
 run6c(const char *src, char **out)
@@ -16,7 +17,7 @@ run6c(const char *src, char **out)
 	snprintf(path, sizeof path, "/tmp/wwt_%d.ww", getpid());
 	FILE *f = fopen(path, "wb");
 	if (f == NULL) return -1;
-	fputs(src, f);
+	wwtest_fputs(src, f);
 	fclose(f);
 
 	char cmd[256];
@@ -60,7 +61,7 @@ static const struct row rows[] = {
 	{ "fn cond(x: i32) i32 = { if (x > 0) { return 1; }; return 0; };",
 	  "CMPQ\t$0, AX" },
 	{ "fn lo() void = { for (let i: i32 = 0; i < 10; i += 1) { }; };", "JMP" },
-	{ "fn callit() i32 = { return 1; };", "TEXT callit,$0" },
+	{ "fn callit() i32 = { return 1; };", "TEXT main.callit,$0" },
 };
 
 int