nocc: stage-0 bootstrap path (binaries gitignored)

Sets up the cc-free fresh-checkout flow without yet committing the
stage-0 binaries. The pieces are in place; flipping the switch is
one `git add -f` away when the compiler is judged stable.

  bootstrap/<arch>/{ww,6c,6a,6l}   stage-0 binaries (gitignored)
  bootstrap/README.md              layout + workflow

Two new targets:

  make bootstrap-snapshot   populates bootstrap/$(ARCH)/ from the
                            currently-built wwstage
  make nocc                 starts from bootstrap/$(ARCH)/, rebuilds
                            the wwstage from source, gates on
                            stage-0 == rebuilt byte-for-byte. Lands
                            in $(OUT)/nocc/ so $(OUT)/ stays
                            untouched. cc is never invoked.

The fixed-point gate has the same shape as `make bootstrap`, just
the entry point flipped: that target trusts cstage; nocc trusts
the checked-in (or local-snapshot) stage-0 binaries.

Implementation notes:

- libwwrt.a is assembled with stage-0 6a, members ordered to match
  $(RT_OBJ) so the embedded symbol table reproduces the cstage
  build byte-for-byte.
- The driver's default lib path is $self_dir/../../lib, which under
  $(NOCC_BIN) resolves to $(NOCC_OUT)/lib (libwwrt only). Each
  ww-build invocation passes -I $(CURDIR)/lib so `use os` etc.
  resolve to source.
- Stage-0 binaries are duplicated under both natural names (ww/6c/
  6a/6l) and _ww-suffixed names so the wwstage driver's
  hard-coded join_path_lit(self_dir, "6c_ww") still finds them.
  When cmd/wwc/ gets deleted at v1.0, that duplication and the
  _ww suffix in the driver both go away.

bootstrap/README.md

@@ -0,0 +1,35 @@
+# bootstrap/
+
+Stage-0 binaries for cold-start builds without `cc`. One subdirectory
+per supported architecture; each holds four statically-linked ELF
+executables that match the wwstage layout:
+
+    bootstrap/amd64/
+        ww      driver, equivalent to out/bin/ww_ww
+        6c      compiler,    out/bin/6c_ww
+        6a      assembler,   out/bin/6a_ww
+        6l      linker,      out/bin/6l_ww
+
+The binaries are intentionally **gitignored** until the compiler stops
+churning enough to make the trust surface stable — see `PLAN.md` and
+the v1.0 plan in `BOOTSTRAP.md`. To put them under your local working
+copy, run `make bootstrap-snapshot` after a green `make wwstage`; to
+ship them, `git add -f bootstrap/amd64/{ww,6c,6a,6l}` explicitly.
+
+## Workflow
+
+A fresh-checkout flow that never invokes `cc`:
+
+    make nocc            # uses bootstrap/<arch>/* as the stage-0 entry
+                         # and proves it reproduces itself byte-for-byte
+
+The non-nocc default (`make`) still drives `cc → cstage → wwstage`.
+Both paths coexist — `make nocc` lands in `out/nocc/` so the cstage
+build under `out/` is undisturbed.
+
+## When to refresh the snapshot
+
+The stage-0 binaries are frozen at the cgen state they were built
+with. A `make nocc` failure usually means the live `selfhost/cmd/wwc`
+cgen has drifted from the snapshot — regenerate with
+`make bootstrap-snapshot` and re-verify.