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".

rt/ensure.ww

@@ -14,10 +14,13 @@
 // no per-type wrapper functions (appendu8 / appendi64) needed.
 //
 // User code never `use`s this — the symbol is resolved at link time
-// from libwwrt.a, like rt_malloc and rt_streq. No `module` declaration:
-// rt/ensure.ww is compiled standalone via `w6c rt/ensure.ww` (not
-// through the driver), and its `export fn rt_ensure` must keep its
-// bare symbol name so the linker resolves it.
+// from libwwrt.a, like rt_malloc and rt_streq. rt/ensure.ww is compiled
+// standalone via `w6c rt/ensure.ww` (not through the driver). Under
+// strict-package (#24a) it declares `package rt;` (matching its
+// directory) like every primary section; the link-resolved name is
+// pinned bare via @symbol on the export (the clause would otherwise
+// mangle it to rt.rt_ensure).
+package rt;
 
 @symbol("rt_malloc") fn malloc(n: u64) *void;
 
@@ -31,7 +34,7 @@ type slice = struct {
 	cap: i64,
 };
 
-export fn rt_ensure(s: *slice, membsz: u64) void = {
+@symbol("rt_ensure") export fn rt_ensure(s: *slice, membsz: u64) void = {
 	if (s.cap >= s.len) { return; };
 	let nc: i64 = s.cap * 2i64;
 	if (nc < 8i64) { nc = 8i64; };