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.

PLAN.md

@@ -291,6 +291,31 @@ Exit criteria:
 - The C `cmd/wwc/`, `cmd/6c/`, `cmd/6a/`, `cmd/6l/` C trees are
   deleted in this phase's final commit.
 
+### Status (2026-05)
+
+Steps 1–6 done. The four ww-side tools live in `selfhost/cmd/`:
+
+- `wwc/` — ww-native lex/parse/check/cgen
+- `6a/`  — ww-native amd64 assembler
+- `6l/`  — ww-native amd64 linker
+- `ww/`  — ww-native driver
+
+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.
+
+Exit criterion 1 (bootstrap green) is met. Exit criterion 2 (delete
+the C trees) is **deferred to v1.0**: removing `cmd/wwc/`, `cmd/6c/`,
+`cmd/6a/`, `cmd/6l/`, `cmd/ww/` is a one-way door. Until the compiler
+stops churning we keep Cstage as the canonical fresh-checkout entry
+point and treat `selfhost/cmd/*` as the path forward. At v1.0, the
+plan is to ship a checked-in stage-0 binary archive
+(`bootstrap/<arch>/{ww,6c,6a,6l}`), delete the C trees, and promote
+`selfhost/cmd/*` to `cmd/*`.
+
+See `BOOTSTRAP.md` for the build flow and `make cstage`/`make wwstage`
+targets.
+
 ## Phase 11 — CSP
 
 Goal: channels and lightweight processes, without GC.