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/check.c

@@ -404,7 +404,17 @@ resolve_type(Checker *c, Node *n)
 			}
 			Tparam *tp = amalloc(c->a, sizeof *tp);
 			tp->name = p->str;
-			tp->type = resolve_type(c, p->lhs);
+			Type *pt = resolve_type(c, p->lhs);
+			/* Hare-style `T...` (marked on the param node via
+			 * Node.op == TK_ELLIPSIS): the param's effective type
+			 * inside the callee is []T, and call sites either
+			 * gather N args of type T or forward an `xs...` slice. */
+			if (p->op == TK_ELLIPSIS) {
+				tp->variadic = 1;
+				tp->type = type_slice(c->a, pt);
+			} else {
+				tp->type = pt;
+			}
 			if (head == NULL) head = tp;
 			else tail->next = tp;
 			tail = tp;
@@ -962,12 +972,38 @@ cexpr(Checker *c, Node *n)
 					err(c, n->pos, "too many arguments");
 				continue;
 			}
+			/* Hare-style variadic param: every remaining arg either
+			 *   - flows into the gather (assignable to element T), or
+			 *   - is a single `xs...` spread of `[]T` (forwarding).
+			 * Don't advance p — the variadic slot absorbs the tail. */
+			if (p->variadic) {
+				Type *elem = (p->type && p->type->kind == TY_SLICE)
+				    ? p->type->sub : ty_err;
+				if (a->kind == N_SPREAD) {
+					if (at != ty_err && p->type != ty_err &&
+					    !type_assignable(p->type, at))
+						err(c, a->pos,
+						    "spread arg: %s not assignable to %s",
+						    type_name(c->a, at),
+						    type_name(c->a, p->type));
+					if (a->next != NULL)
+						err(c, a->pos,
+						    "spread arg must be the last");
+				} else if (elem != ty_err && at != ty_err) {
+					if (!type_assignable(elem, at))
+						err(c, a->pos,
+						    "variadic arg: %s not assignable to %s",
+						    type_name(c->a, at),
+						    type_name(c->a, elem));
+				}
+				continue;
+			}
 			if (!type_assignable(p->type, at) && at != ty_err && p->type != ty_err)
 				err(c, a->pos, "argument type %s not assignable to %s",
 				    type_name(c->a, at), type_name(c->a, p->type));
 			p = p->next;
 		}
-		if (p != NULL)
+		if (p != NULL && !p->variadic)
 			err(c, n->pos, "not enough arguments");
 		return n->type = u->ret ? u->ret : ty_void;
 	}
@@ -1514,7 +1550,14 @@ build_fn_type(Checker *c, Node *fn)
 		}
 		Tparam *tp = amalloc(c->a, sizeof *tp);
 		tp->name = p->str;
-		tp->type = resolve_type(c, p->lhs);
+		Type *pt = resolve_type(c, p->lhs);
+		/* Hare-style `T...` — see resolve_type N_TFN. */
+		if (p->op == TK_ELLIPSIS) {
+			tp->variadic = 1;
+			tp->type = type_slice(c->a, pt);
+		} else {
+			tp->type = pt;
+		}
 		if (head == NULL) head = tp;
 		else tail->next = tp;
 		tail = tp;