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.

lib/os/os.ww

@@ -18,19 +18,21 @@ export fn assert(cond: bool, msg: str) void = {
 	if (!cond) { abort(msg); };
 };
 
-def SYS_READ:   i64 = 0;
-def SYS_WRITE:  i64 = 1;
-def SYS_OPEN:   i64 = 2;
-def SYS_CLOSE:  i64 = 3;
-def SYS_LSEEK:  i64 = 8;
-def SYS_ACCESS: i64 = 21;
-def SYS_DUP2:   i64 = 33;
-def SYS_GETPID: i64 = 39;
-def SYS_FORK:   i64 = 57;
-def SYS_EXECVE: i64 = 59;
-def SYS_EXIT:   i64 = 60;
-def SYS_WAIT4:  i64 = 61;
-def SYS_UNLINK: i64 = 87;
+def SYS_READ:        i64 = 0;
+def SYS_WRITE:       i64 = 1;
+def SYS_OPEN:        i64 = 2;
+def SYS_CLOSE:       i64 = 3;
+def SYS_LSEEK:       i64 = 8;
+def SYS_ACCESS:      i64 = 21;
+def SYS_DUP2:        i64 = 33;
+def SYS_GETPID:      i64 = 39;
+def SYS_FORK:        i64 = 57;
+def SYS_EXECVE:      i64 = 59;
+def SYS_EXIT:        i64 = 60;
+def SYS_WAIT4:       i64 = 61;
+def SYS_UNLINK:      i64 = 87;
+def SYS_GETCWD:      i64 = 79;
+def SYS_GETDENTS64:  i64 = 217;
 
 // open(2) flags. Linux values, matching <fcntl.h>.
 def O_RDONLY: i32 = 0;
@@ -65,7 +67,7 @@ export fn close(fd: i32) i32 = {
 
 // dup2(2): make `newfd` refer to the same description as `oldfd`,
 // closing `newfd` first if open. Returns `newfd` on success or a
-// negative errno. Used by 6c_ww to redirect stdout into an output
+// negative errno. Used by w6c_ww to redirect stdout into an output
 // file without changing the cgen emit path.
 export fn dup2(oldfd: i32, newfd: i32) i32 = {
 	return syscall2(SYS_DUP2, oldfd: i64, newfd: i64): i32;
@@ -178,3 +180,29 @@ export fn wait4(pid: i32, status_out: *i32, options: i32, rusage: *void) i32 = {
 	return syscall4(SYS_WAIT4, pid: i64, status_out: i64,
 	    options: i64, rusage: i64): i32;
 };
+
+// getcwd(2) — Linux flavour. Writes the NUL-terminated cwd into `buf`
+// and returns the number of bytes written (including the NUL), or a
+// negative errno. The driver uses it to expand `.` to the cwd's
+// basename for `ww build` / `ww test`.
+export fn getcwd(buf: *u8, n: u64) i64 = {
+	return syscall2(SYS_GETCWD, buf: i64, n: i64);
+};
+
+// getdents64(2) — Linux directory enumeration. The fd must be opened
+// with O_RDONLY on a directory. `buf` receives a packed sequence of
+// linux_dirent64 records:
+//
+//   struct linux_dirent64 {
+//       u64  d_ino;       // 0..7
+//       i64  d_off;       // 8..15
+//       u16  d_reclen;    // 16..17  — total bytes for this record
+//       u8   d_type;      // 18      — DT_REG/DT_DIR/...
+//       u8   d_name[];    // 19..    — NUL-terminated name + padding
+//   };
+//
+// Returns bytes written into `buf` (advance by d_reclen to walk),
+// 0 at end-of-directory, or a negative errno.
+export fn getdents64(fd: i32, buf: *u8, n: u64) i64 = {
+	return syscall3(SYS_GETDENTS64, fd: i64, buf: i64, n: i64);
+};