ww: import toolchain — C bootstrap + ww-side self-host (phases 0-10)

C bootstrap (phases 0-9):
  cmd/wwc, cmd/6c, cmd/6a, cmd/6l, cmd/ww, rt, lib/*.

ww-side self-host (phase 10):
  selfhost/cmd/wwc — ww-cgen frontend; bootstrap fixed point.
  selfhost/cmd/6a  — assembler; byte-identical to C 6a (test 991).
  selfhost/cmd/6l  — linker w/ archive (.a) support; byte-identical
                     to C 6l (test 992).
  selfhost/cmd/ww  — driver (build/run/version); byte-identical to
                     C ww (test 993).

make test: 15/15. make bootstrap: ww2.s == ww3.s, ww2.o == ww3.o,
ww2 == ww3 byte-identical, with the full ww-tooled chain.

test/run

@@ -0,0 +1,66 @@
+#!/bin/sh
+# test/run — driver for `make test`. Plan 9 rc-flavoured but plain sh.
+#
+# Walks the test/ tree:
+#   test/wwc/<NNN>_<name>.c → out/bin/test_<name> binary, run it.
+#   test/lang/<phase>/*.ww  → compile/run via $WW, compare stdout/exit
+#                              with header comments (// expected: ...).
+# Exit non-zero on first failure.
+
+set -e
+
+BIN=${BIN:-out/bin}
+WW=${WW:-$BIN/ww}
+
+fail=0
+ran=0
+
+# ---- C-side unit tests --------------------------------------------------
+for t in test/wwc/*.c; do
+	[ -f "$t" ] || continue
+	name=${t##*/}
+	name=${name%.c}
+	# strip leading <NNN>_
+	short=${name#[0-9][0-9][0-9]_}
+	bin=$BIN/test_$short
+	if [ ! -x "$bin" ]; then
+		echo "SKIP $name (no binary $bin)"
+		continue
+	fi
+	if "$bin" >"$BIN/.$short.out" 2>"$BIN/.$short.err"; then
+		echo "ok   $name"
+	else
+		rc=$?
+		echo "FAIL $name (rc=$rc)"
+		echo "--- stdout ---"
+		cat "$BIN/.$short.out"
+		echo "--- stderr ---"
+		cat "$BIN/.$short.err"
+		fail=$((fail + 1))
+	fi
+	ran=$((ran + 1))
+done
+
+# ---- ww source-level tests ----------------------------------------------
+# Each test/lang/<phase>/*.ww has a header:
+#   // expected-exit: 0
+#   // expected-stdout: hello world
+# These run with `ww run`; phases that don't compile yet skip entries
+# that begin with `// phase: <N>` if N > current.
+if [ -d test/lang ]; then
+	for f in test/lang/*/*.ww; do
+		[ -f "$f" ] || continue
+		# Skip until ww run exists; placeholder for phase 7+
+		echo "skip $f (ww run not online yet)"
+	done
+fi
+
+if [ $ran -eq 0 ]; then
+	echo "no tests were run"
+	exit 1
+fi
+if [ $fail -gt 0 ]; then
+	echo "$fail test(s) failed"
+	exit 1
+fi
+echo "all $ran tests passed"