Seed updates and ./wbuild update discipline

The seed ./w is a statically linked 32-bit x86 Linux ELF that
compiles the compiler's own sources. It is not committed: SEEDS
at the repo root pins a release tag, asset name and sha256 for each
seed (w, w_darwin, w.exe), and ./wbuild downloads a missing
seed from that GitHub release, refusing to run it on a hash mismatch.

Bootstrap chain: the seed compiles w.w into bin/wv2; ./wbuild build continues wv2 -> wv3 -> wv4 -> wv5 (those self-host stages
compile with --strict, so any warning fails the build), and
./wbuild verify asserts the fixpoint wv3 == wv4 == wv5.

Local promotion: ./wbuild update

./wbuild update archives the current seed (tools/promote_seed.w -> old/)
and promotes the bin/wv3 fixpoint onto ./w (update_darwin /
update_win do the same for the other seeds). Rules:

• Only after ./wbuild verify passes. The target depends on
  verify, so a broken fixpoint cannot be promoted — never work
  around that ordering.
• Local only. A promoted ./w changes nothing for other checkouts
  or CI, which keep bootstrapping from the SEEDS pin. Afterwards
  wbuild prints a one-line notice that the seed differs from its
  pin; rm w re-downloads the pinned seed to get back to baseline.

Publishing a seed: release tag + SEEDS bump

Making a new seed real for everyone is a release, not an update
(details in docs/release.md):

1. Land the feature PR. It must build under the current pinned seed.
2. Cut a release at that commit (version bumped in package.wmeta and
   w.w's --version string; the workflow verifies every fixpoint).
3. In a follow-up PR, bump every SEEDS line to the new tag,
   copying sha256 values from the release's SHA256SUMS. The
   single-tag pin keeps all seeds compiling the same sources by
   construction.
4. Only after the SEEDS bump lands may seed-compiled sources use the
   new syntax.

The no-post-seed-syntax rule

Everything in w.w's transitive import graph is compiled by the
pinned seed: compiler/, grammar/, code_generator/, debugger/,
the auto-imported container runtime (structures/hash_table.w,
structures/w_list.w), libs/extras/{c_import,c_preprocessor, parser_generator}, and any lib/ file those import. None of it may
use language syntax newer than the seed until the SEEDS bump lands —
a feature can work in bin/wv2 yet break the next cold bootstrap.
./bin/wv2 deps w.w prints the exact membership; new syntax is fine
in tests/ and other leaf consumers once bin/wv2 exists.