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/w6c/gc.h

@@ -0,0 +1,56 @@
+/*
+ * gc.h — w6c-private header: Prog/Adr structs, scratch register set,
+ * stack-frame state. Plan 9 cmd/6c/gc.h shape, trimmed.
+ */
+#ifndef SIX_GC_H
+#define SIX_GC_H
+
+#include "ww.h"
+#include "6.out.h"
+
+typedef struct Prog Prog;
+typedef struct Adr  Adr;
+
+/* one operand: register, immediate, indirect, or symbolic. */
+struct Adr {
+	int	type;		/* D_AX, D_CONST, D_INDIR, ... */
+	int	reg;		/* base register for D_INDIR */
+	long long offset;	/* immediate value or memory displacement */
+	const char *sym;	/* symbol name for D_EXTERN/D_BRANCH */
+};
+
+struct Prog {
+	int	as;		/* opcode (A_MOVQ, ...) */
+	Adr	from;		/* source operand */
+	Adr	to;		/* destination operand (Plan 9 order) */
+	int	line;
+	const char *label;	/* if non-NULL, this prog is preceded by label: */
+	Prog	*link;
+};
+
+/* per-fn codegen state */
+typedef struct Cg Cg;
+struct Cg {
+	Arena	*a;
+	Prog	*head, *tail;
+	const char *fnname;
+	int	framesize;	/* bytes of locals; 16-byte aligned */
+	int	curoff;		/* current top of locals */
+	Scope	*locals;	/* (name → offset) tracked via Sym */
+	int	labelseq;
+};
+
+/* cgen.c */
+void	cg_init(Cg*, Arena*);
+void	cg_file(Cg*, FILE *out, Node *file);
+Prog	*newprog(Cg*, int op);
+void	emit(Cg*, Prog*);
+
+/* txt.c */
+void	txt_emit(FILE*, Prog *head);
+
+/* swt.c, peep.c, reg.c — placeholders for now */
+void	peephole(Cg*);
+void	regalloc_init(Cg*);
+
+#endif