build: split cstage/wwstage targets, document bootstrap

Phase 10 step 8 (delete the C trees) is deferred to v1.0 — until the
compiler stops churning we keep Cstage as the fresh-checkout entry
point. Split the Makefile so the two stages are named, and add
BOOTSTRAP.md describing the cstage → ww1 → ww2 → ww3 fixed-point
flow. PLAN.md gets a status note pointing at it.

BOOTSTRAP.md

@@ -0,0 +1,70 @@
+# BOOTSTRAP
+
+How `ww` builds itself.
+
+`ww` is self-hosted: the ww-side compiler/assembler/linker/driver live
+under `selfhost/cmd/` and reach a byte-identical fixed point under
+`make bootstrap`. But the *first* ww binary on a fresh checkout has to
+come from somewhere. Today, that "somewhere" is a small C bootstrap
+toolchain — the **Cstage**:
+
+    cmd/wwc/   frontend library (lex, parse, check)  → libwwc.a
+    cmd/6c/    amd64 compiler   .ww → .s
+    cmd/6a/    amd64 assembler  .s  → .o
+    cmd/6l/    amd64 linker     .o  → static ELF
+    cmd/ww/    user-facing driver (orchestrates 6c → 6a → 6l)
+
+Cstage is built with `cc`. Once it exists, it compiles the
+ww-rewritten tools (the **wwstage**) under `selfhost/cmd/`, which then
+self-compile to a byte-identical fixed point. After that, no C is
+involved.
+
+## Build flow
+
+    cstage     (cc → C tools, one-time)
+      │
+      ├──→ ww1   wwstage tools, built by Cstage
+      │     │
+      │     └──→ ww2   wwstage self-compiles
+      │            │
+      │            └──→ ww3   ww2 self-compiles
+      │                   │
+      │                   ↓
+      │              cmp ww2 ww3   →   fixed point ✓
+      ↑
+      └ from here on, no C is invoked
+
+## Make targets
+
+    make             builds cstage + wwstage + libs
+    make cstage      Cstage only (cc → ww, 6c, 6a, 6l, wwdump, libs)
+    make wwstage     wwstage only (assumes cstage)
+    make bootstrap   three-stage build; gates on cmp ww2 == ww3
+    make test        runs all tests, including the 990–993 diffs
+                     (ww-side tools vs C-side tools, byte-for-byte)
+
+## Status
+
+Phase 10 of `PLAN.md` is at its first exit criterion: the bootstrap is
+green and `selfhost/cmd/{wwc,6a,6l,ww}` are byte-identical to their
+Cstage counterparts. The second exit criterion — deleting the C trees
+— is **deferred to v1.0**: until the compiler stops churning, the
+Cstage stays as the fresh-checkout entry point. A `git clone` today
+still requires `cc`.
+
+## What "no C" looks like
+
+When the compiler stabilises:
+
+1. Cross-compile or natively build a known-good wwstage on each
+   supported arch.
+2. Ship those binaries under `bootstrap/<arch>/{ww,6c,6a,6l}` in the
+   tree. They are the new stage 0.
+3. Delete `cmd/wwc/`, `cmd/6c/`, `cmd/6a/`, `cmd/6l/`, `cmd/ww/`.
+4. Move `selfhost/cmd/*` to `cmd/*`.
+5. Build path: stage-0 binary → ww1 → ww2 → fixed point, same as
+   today, just without `cc`.
+
+This is the standard self-hosting story (Go, Rust, OCaml all do
+variants of it). The bootstrap binaries become the new "trusting
+trust" surface.