ww: rename toolchain to w-prefix + hare-style build/run/test driver

Plan 9-style w-prefix on the per-arch tools, disambiguating from the
real Plan 9 6c/6a/6l in ref/plan9front/:

    cmd/wwc/      → cmd/wcc/        libwwc.a → libwcc.a
    cmd/6{c,a,l}  → cmd/w6{c,a,l}   binary names too
    test/wwc/     → test/wcc/       6 test files w/ w6 prefix
    selfhost/cmd  mirror in lockstep
    bootstrap/amd64/{w6c,w6a,w6l}   snapshot binaries (gitignored)
    WW_6{C,A,L}   → WW_W6{C,A,L}    env-var overrides

Plan 9 source-tree refs ("Plan 9 6c shape", ref/plan9front/, etc.)
preserved. Hare-style driver, both C and ww sides:

    ww test [path]   discover *_test.ww in a directory module, run
                     each; single-file mode for `ww test foo.ww`
    Module-by-name   `ww build foo` resolves to foo.ww or foo/foo.ww
                     via search path (cwd : -I dirs : $WW_LIB)
    Default-to-cwd   `ww build` / `ww test` build the cwd module
    Run pass-through `ww run path arg1 arg2` reaches the program

lib/os: getcwd (79) and getdents64 (217) syscalls power `.` resolution
and directory enumeration on the ww side.

Makefile: wwstage tool deps now include lib/os/os.ww (+ lib/strconv
for wwdump_ww) so lib/* edits force their rebuild instead of leaving
stale binaries — surfaced when test 995 first failed against a stale
w6c_ww built before the lib/os additions.

Test 993 byte-identical parity gate (C-side ww vs ww-side ww_ww on a
build corpus) stays green; all 19 tests pass.

BOOTSTRAP.md

@@ -8,11 +8,11 @@ under `selfhost/cmd/` and reach a byte-identical fixed point under
 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)
+    cmd/wcc/   frontend library (lex, parse, check)  → libwcc.a
+    cmd/w6c/    amd64 compiler   .ww → .s
+    cmd/w6a/    amd64 assembler  .s  → .o
+    cmd/w6l/    amd64 linker     .o  → static ELF
+    cmd/ww/    user-facing driver (orchestrates w6c → w6a → w6l)
 
 Cstage is built with `cc`. Once it exists, it compiles the
 ww-rewritten tools (the **wwstage**) under `selfhost/cmd/`, which then
@@ -37,7 +37,7 @@ involved.
 ## Make targets
 
     make                builds cstage + wwstage + libs
-    make cstage         Cstage only (cc → ww, 6c, 6a, 6l, wwdump, libs)
+    make cstage         Cstage only (cc → ww, w6c, w6a, w6l, wwdump, libs)
     make wwstage        wwstage only (assumes cstage)
     make bootstrap      three-stage build; gates on cmp ww2 == ww3
     make test           runs all tests, including 990–995 (ww-side
@@ -57,12 +57,12 @@ Without cc:
 ## Status
 
 Phase 10 of `PLAN.md` is at its first exit criterion: the bootstrap is
-green and `selfhost/cmd/{wwc,6c,6a,6l,ww}` are byte-identical to their
+green and `selfhost/cmd/{wcc,w6c,w6a,w6l,ww}` are byte-identical to their
 Cstage counterparts. Test 995 pins the stronger property — the
-wwstage rebuilds every one of its own tools through `ww_ww + 6c_ww +
-6a_ww + 6l_ww`, byte-for-byte. Test 996 pins the equivalent for the
-dynamic linker: `6l_ww` with `-L`/`-l` produces a byte-identical
-PT_INTERP+PT_DYNAMIC binary to C-side `6l` on snake.
+wwstage rebuilds every one of its own tools through `ww_ww + w6c_ww +
+w6a_ww + w6l_ww`, byte-for-byte. Test 996 pins the equivalent for the
+dynamic linker: `w6l_ww` with `-L`/`-l` produces a byte-identical
+PT_INTERP+PT_DYNAMIC binary to C-side `w6l` on snake.
 
 `make nocc` is wired and verifies stage-0 self-reproduction locally;
 the stage-0 binaries under `bootstrap/$(ARCH)/` are gitignored until
@@ -79,12 +79,12 @@ The pieces are already wired; only the binary commit is held back.
 When the compiler stabilises:
 
 1. On each supported arch, run `make wwstage && make bootstrap-snapshot`
-   to populate `bootstrap/<arch>/{ww,6c,6a,6l}` from the current
+   to populate `bootstrap/<arch>/{ww,w6c,w6a,w6l}` from the current
    wwstage. Verify with `make nocc`.
-2. `git add -f bootstrap/<arch>/{ww,6c,6a,6l}` — explicit because
+2. `git add -f bootstrap/<arch>/{ww,w6c,w6a,w6l}` — explicit because
    the dir is gitignored. The binary SHAs become the new trust
    surface.
-3. Delete `cmd/wwc/`, `cmd/6c/`, `cmd/6a/`, `cmd/6l/`, `cmd/ww/`.
+3. Delete `cmd/wcc/`, `cmd/w6c/`, `cmd/w6a/`, `cmd/w6l/`, `cmd/ww/`.
    Drop `CC`/`AR` C-tool dependencies from the top-level Makefile.
 4. Move `selfhost/cmd/*` to `cmd/*` and `make nocc` becomes the
    default `make`.