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.

.gitignore

@@ -0,0 +1,12 @@
+# Build outputs.
+/out/
+
+# Stray executables produced by ad-hoc `ww build` runs in the project
+# root. Real source for these lives under selfhost/test/ as .ww files.
+/loop
+/sym_link
+
+# Per-module build artifacts. The .combined.ww files under selfhost/
+# are intentionally tracked — they're frozen bootstrap inputs.
+selfhost/**/*.o
+selfhost/**/*.s