w6c+selfhost+lib: Hare-style variadic call sites

Param-decl `name: T...` (Tparam.variadic=1, type []T), call-site
gather of N args into a fresh `[N]T`, forward via `xs...`, full
selfhost mirror, and lib/fmt graduated to the Hare shape.

Frontend:
  - parse: `T...` after a param's type stamps Node.op=TK_ELLIPSIS
    and breaks out (variadic must be last).
  - check: resolve_type N_TFN / build_fn_type wrap the param type
    as []T and set tp->variadic. N_CALL accepts either a tail of
    args assignable to T (gather) or a single `xs...` spread of
    []T (forward); both bypass the "too many args" check on the
    variadic slot.
  - type: type_eq compares Tparam.variadic.

Cgen (cstage):
  - call site: when the callee has a variadic last param,
    materialise the tail args into a frame-resident `[N]T` via
    localoff, write a 24B slice descriptor (ptr,len,cap), and
    splice a synthesised N_IDENT into args[] so the downstream
    widen/eval/pop loops see one slice slot. Tagged-element types
    route each store through cg_widen_tagged_store. Forwarding
    skips gather: the N_SPREAD wrapper is replaced with its inner
    slice expression. Empty form writes {nil,0,0}. args[] / widen[]
    bump from 16 to 64 to accommodate Hare's mixed-arg printers.

Selfhost mirror:
  - lib/ww/parse: `T...` mark on N_PARAM.op.
  - cgen: varargseq counter on Cg; scanlocals reserves
    @vararg_d_N + @vararg_sl_N per variadic call (seq recorded on
    N_CALL.uval so cgcall picks the same names). cgcall does the
    same gather/forward and N_IDENT splice. cgfnparams treats
    variadic params as 24B slice slots via a synthesised TSLICE
    tnode. pushargsrev skips the tagged-widen detection for
    variadic params (effective type is []T, not tagged).
  - rhstargetname now recognises N_TRUE/N_FALSE/N_RUNELIT and
    typed N_INTLIT so the variant-tag lookup finds bool/rune/iN
    variants instead of falling through to "first non-str" (which
    misassigned tag 0 to bool in tagged unions like formattable).

lib/fmt graduated: print/println/fprint/fprintln/errorln/fatal
take `args: formattable...`. Bare `error` (no -ln) is skipped —
the leaf name collides with strconv's `type error = !(invalid |
overflow)` under the driver's flat namespace.

Tests: 5 new e2e rows (plain gather, zero-arg, tagged element,
forwarding, fmt.println end-to-end). lib/CLAUDE.md workaround
paragraph replaced with the Hare-shape description.

cmd/wcc/parse.c

@@ -135,9 +135,18 @@ parseparams(Parser *p)
 			n->strlen = 0;
 			n->lhs = parsetype(p);
 		}
+		/* Hare-style variadic: `name: T...`. The trailing `...`
+		 * after the type promotes the param's type to []T at type-
+		 * resolution time; call sites gather N args or forward a
+		 * `xs...` spread. Marked on n->op so check.c and selfhost
+		 * recognise it without needing a new Node kind. */
+		if (accept(p, TK_ELLIPSIS))
+			n->op = TK_ELLIPSIS;
 		if (head == NULL) head = n;
 		else tail->next = n;
 		tail = n;
+		if (n->op == TK_ELLIPSIS)
+			break;	/* Hare-style variadic must be the last param */
 		if (!accept(p, TK_COMMA))
 			break;
 		if (p->cur.kind == TK_RPAREN)	/* trailing comma */