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".
Cstage and wwstage share the latent: check.c's N_INDEX bespoke
TY_PTR-over-TY_SLICE clause peeled the slice in `*[]T[i]` and
returned the element of the element, while wwstage's elemsizeof
had no N_TSLICE arm for the post-N_TPTR-peel elem and fell to
the 8B catch-all. Splitting leaves one stage broken on the
exact `*[]T[i]` shape the new 754 sentinel asserts byte-identical
between stages (rule 11). The companion 24B per-element copy
emit is a separate codegen wedge already pinned inline at
cmd/w6c/cgen.c:6518; out-of-scope here and noted in the fixture
header.
