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/w6a/a.h

@@ -0,0 +1,107 @@
+/*
+ * a.h — w6a-private header. Modelled on Plan 9 cmd/6a/a.h, trimmed
+ * to the instruction subset that w6c emits.
+ *
+ * w6a is line-oriented and has no preprocessor: each non-blank, non-
+ * label line is one instruction. We read the whole file into a list
+ * of `Aprog`s, then encode and emit ELF64.
+ */
+#ifndef SIX_A_H
+#define SIX_A_H
+
+#include "6.out.h"
+#include <stddef.h>
+#include <stdint.h>
+#include <stdio.h>
+
+typedef int8_t   i8;
+typedef int16_t  i16;
+typedef int32_t  i32;
+typedef int64_t  i64;
+typedef uint8_t  u8;
+typedef uint16_t u16;
+typedef uint32_t u32;
+typedef uint64_t u64;
+
+typedef struct Aoperand Aoperand;
+typedef struct Aprog    Aprog;
+typedef struct Asym     Asym;
+typedef struct Areloc   Areloc;
+typedef struct Asm      Asm;
+
+struct Aoperand {
+	int	type;	/* D_NONE, D_AX..D_R15, D_CONST, D_INDIR, D_EXTERN, D_BRANCH */
+	int	reg;	/* base register for D_INDIR */
+	i64	offset;	/* immediate or displacement */
+	const char *sym;
+};
+
+struct Aprog {
+	int	as;	/* opcode (A_*) */
+	Aoperand from;
+	Aoperand to;
+	int	line;
+	const char *label;	/* label preceding this prog, if any */
+	Aprog	*link;
+	/* for A_DATA: raw payload bytes interned by the parser */
+	u8	*bytes;
+	u64	nbytes;
+};
+
+struct Asym {
+	const char *name;
+	int	defined;	/* 1 if we own its address */
+	int	is_text;	/* if 1, address is in .text */
+	int	is_global;	/* exported (TEXT) */
+	u64	addr;		/* offset within section if defined */
+	int	idx;		/* ELF symtab index, filled at emit time */
+	Asym	*next;
+};
+
+struct Areloc {
+	u64	off;	/* offset within .text where relocation lands */
+	int	kind;	/* R_X86_64_PLT32 (4), R_X86_64_PC32 (2) */
+	Asym	*sym;
+	i64	addend;
+	Areloc	*next;
+};
+
+struct Asm {
+	/* parser state */
+	const char *file;
+	const char *src;
+	u64	srclen;
+	u64	pos;
+	int	line;
+
+	/* program list */
+	Aprog	*head, *tail;
+
+	/* output text section */
+	u8	*text;
+	u64	textcap, textlen;
+
+	/* symbols */
+	Asym	*syms;
+	Areloc	*relocs;
+
+	int	errs;
+};
+
+/* lex.c / parse.c */
+void	a_init(Asm*, const char *file, const char *src, u64 len);
+int	a_parse(Asm*);
+
+/* asm.c */
+int	a_encode(Asm*);
+
+/* obj.c */
+int	a_emit_elf(Asm*, FILE *out);
+
+/* helpers */
+Asym	*a_intern(Asm*, const char *name);
+void	a_emit_byte(Asm*, u8);
+void	a_emit_u32(Asm*, u32);
+void	a_addreloc(Asm*, u64 off, int kind, Asym *s, i64 add);
+
+#endif