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.

cmd/wcc/mem.c

@@ -0,0 +1,117 @@
+/*
+ * mem.c — arena allocator. No free per allocation; freearena releases
+ * the whole chain. Aligned to 16 so structs with 8-byte fields and
+ * doubles are happy.
+ *
+ * Hot allocations in the compiler land in arenas: tokens, AST nodes,
+ * symbols, types. The chunk size doubles up to a cap so we don't
+ * fragment on huge inputs.
+ */
+#include "ww.h"
+#include <stdlib.h>
+#include <string.h>
+
+#define ALIGN		16
+#define INIT_CHUNK	(64 * 1024)
+#define MAX_CHUNK	(4 * 1024 * 1024)
+
+static u64
+roundup(u64 n, u64 a)
+{
+	return (n + a - 1) & ~(a - 1);
+}
+
+Arena *
+newarena(void)
+{
+	Arena *a = calloc(1, sizeof *a);
+	if (a == NULL)
+		fatal("newarena: out of memory");
+	a->buf = malloc(INIT_CHUNK);
+	if (a->buf == NULL)
+		fatal("newarena: out of memory");
+	a->cap = INIT_CHUNK;
+	return a;
+}
+
+static void
+grow(Arena *a, u64 need)
+{
+	u64 ncap = a->cap * 2;
+	if (ncap > MAX_CHUNK)
+		ncap = MAX_CHUNK;
+	if (ncap < need)
+		ncap = roundup(need, ALIGN);
+
+	/* push current chunk onto chain, allocate fresh head */
+	Arena *old = malloc(sizeof *old);
+	if (old == NULL)
+		fatal("arena: oom");
+	*old = *a;
+	a->next = old;
+	a->buf = malloc(ncap);
+	if (a->buf == NULL)
+		fatal("arena: oom (chunk=%llu)", (unsigned long long)ncap);
+	a->off = 0;
+	a->cap = ncap;
+}
+
+void *
+amalloc(Arena *a, u64 n)
+{
+	n = roundup(n, ALIGN);
+	if (n > a->cap - a->off)
+		grow(a, n);
+	void *p = a->buf + a->off;
+	a->off += n;
+	a->total += n;
+	memset(p, 0, n);
+	return p;
+}
+
+char *
+astrdup(Arena *a, const char *s)
+{
+	u64 n = strlen(s);
+	char *p = amalloc(a, n + 1);
+	memcpy(p, s, n);
+	return p;
+}
+
+char *
+astrndup(Arena *a, const char *s, u64 n)
+{
+	char *p = amalloc(a, n + 1);
+	memcpy(p, s, n);
+	return p;
+}
+
+char *
+aprintf(Arena *a, const char *fmt, ...)
+{
+	va_list ap;
+	va_start(ap, fmt);
+	int n = vsnprintf(NULL, 0, fmt, ap);
+	va_end(ap);
+	if (n < 0)
+		fatal("aprintf: vsnprintf failed");
+	char *p = amalloc(a, (u64)n + 1);
+	va_start(ap, fmt);
+	vsnprintf(p, (size_t)n + 1, fmt, ap);
+	va_end(ap);
+	return p;
+}
+
+void
+freearena(Arena *a)
+{
+	while (a) {
+		Arena *next = a->next;
+		free(a->buf);
+		/* The head Arena was returned by newarena() and is the only
+		 * one we should free as a struct; the linked older ones were
+		 * allocated by grow() and are also freeable. */
+		free(a);
+		a = next;
+	}
+}