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/w6l/out.c

@@ -0,0 +1,94 @@
+/*
+ * out.c — emit a static ELF64 executable.
+ *
+ * Layout (file order):
+ *   [0..64)       ELF header
+ *   [64..120)     program header (one PT_LOAD)
+ *   [120..0x1000) zero pad
+ *   [0x1000..)    .text bytes
+ *
+ * The single PT_LOAD covers the whole file, R+X. No interpreter,
+ * no dynamic, no .bss yet. Entry point is the address of the
+ * symbol named "_start" (or whatever main supplies via -e).
+ */
+#include "l.h"
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#define ET_EXEC		2
+#define EM_X86_64	62
+#define EV_CURRENT	1
+#define ELFCLASS64	2
+#define ELFDATA2LSB	1
+
+#define PT_LOAD		1
+#define PF_X		1
+#define PF_W		2
+#define PF_R		4
+
+#pragma pack(push, 1)
+typedef struct {
+	u8 e_ident[16];
+	u16 e_type, e_machine;
+	u32 e_version;
+	u64 e_entry, e_phoff, e_shoff;
+	u32 e_flags;
+	u16 e_ehsize, e_phentsize, e_phnum, e_shentsize, e_shnum, e_shstrndx;
+} Ehdr;
+
+typedef struct {
+	u32 p_type, p_flags;
+	u64 p_offset, p_vaddr, p_paddr;
+	u64 p_filesz, p_memsz, p_align;
+} Phdr;
+#pragma pack(pop)
+
+int
+l_emit_elf(Lnk *l, FILE *f, u64 base, u64 entry)
+{
+	/* Dispatch: any loaded shared object plus any dynamic ref means
+	 * we owe the loader a real PT_INTERP/PT_DYNAMIC binary. */
+	if (l->sos != NULL && l->dyn_n > 0)
+		return l_emit_dyn_elf(l, f, base, entry);
+
+	const u64 text_off = 0x1000;
+	const u64 text_va  = base + text_off;
+	const u64 filesz   = text_off + l->textlen;
+
+	Ehdr eh = {0};
+	memcpy(eh.e_ident, "\x7f""ELF", 4);
+	eh.e_ident[4] = ELFCLASS64;
+	eh.e_ident[5] = ELFDATA2LSB;
+	eh.e_ident[6] = EV_CURRENT;
+	eh.e_type = ET_EXEC;
+	eh.e_machine = EM_X86_64;
+	eh.e_version = EV_CURRENT;
+	eh.e_entry = entry;
+	eh.e_phoff = sizeof(Ehdr);
+	eh.e_ehsize = sizeof(Ehdr);
+	eh.e_phentsize = sizeof(Phdr);
+	eh.e_phnum = 1;
+	(void)text_va;
+
+	Phdr ph = {0};
+	ph.p_type = PT_LOAD;
+	ph.p_flags = PF_R | PF_X;
+	ph.p_offset = 0;
+	ph.p_vaddr = base;
+	ph.p_paddr = base;
+	ph.p_filesz = filesz;
+	ph.p_memsz = filesz;
+	ph.p_align = 0x1000;
+
+	fwrite(&eh, 1, sizeof eh, f);
+	fwrite(&ph, 1, sizeof ph, f);
+
+	/* pad to text_off */
+	long here = ftell(f);
+	for (long i = here; i < (long)text_off; i++) fputc(0, f);
+
+	if (l->textlen) fwrite(l->text, 1, l->textlen, f);
+
+	return 0;
+}