ww: driver shells to wwstage tools

build_one now invokes 6c_ww / 6a_ww / 6l_ww from $self_dir, not
the C-built binaries that share the directory. After this change
`ww_ww build foo.ww` touches no cstage code at runtime — the
fresh-checkout cstage is still needed to bring the wwstage into
existence, but day-to-day work runs on the ww toolchain end to
end. The C `ww` driver in cmd/ww/ still drives the C 6c/6a/6l.

Test 993 (which used to be trivial — both drivers invoked the
same C tools) now meaningfully compares the cstage pipeline
against the wwstage pipeline on hello + wwdump and confirms
byte-identical exes.

The .combined.ww files for 6a/6l/ww/wwdump and smoke are
regenerated by the ww driver's `expand()` step; their diff is
the lib/os dup2 wrapper and the cgen.ww port from the prior two
commits, propagating into the bootstrap inputs.

PLAN.md

@@ -293,16 +293,26 @@ Exit criteria:
 
 ### Status (2026-05)
 
-Steps 1–6 done. The four ww-side tools live in `selfhost/cmd/`:
+Steps 1–6 done. The five ww-side tools live in `selfhost/cmd/`:
 
-- `wwc/` — ww-native lex/parse/check/cgen
+- `wwc/` — ww-native lex/parse/check/cgen (the frontend library)
+- `6c/`  — ww-native compiler binary; thin driver over wwc's cgen
 - `6a/`  — ww-native amd64 assembler
 - `6l/`  — ww-native amd64 linker
-- `ww/`  — ww-native driver
+- `ww/`  — ww-native driver, shells to `6c_ww/6a_ww/6l_ww`
 
 Step 7 reaches its fixed point: `make bootstrap` produces ww1 → ww2 →
-ww3 with `cmp ww2 ww3` byte-identical. Tests 990–993 confirm each
-selfhost tool is byte-identical to its Cstage counterpart.
+ww3 with `cmp ww2 ww3` byte-identical. Tests 990–994 confirm each
+selfhost tool is byte-identical to its Cstage counterpart on the
+wwdump-corpus (and `ww_ww build` is byte-identical to `ww build` on
+test 993's hello + wwdump corpus, despite using a different
+toolchain underneath).
+
+`ww_ww build foo.ww` is now C-free at runtime: the driver invokes
+the wwstage tools end-to-end. Cstage is still required at first-
+checkout time (no checked-in stage-0 binary yet) and for examples
+that exercise the C-only features still missing from the ww side:
+ET_DYN dynamic linking in 6l (commit `ecf0a84`, used by `snake`).
 
 Exit criterion 1 (bootstrap green) is met. Exit criterion 2 (delete
 the C trees) is **deferred to v1.0**: removing `cmd/wwc/`, `cmd/6c/`,