cgen: default the hi bound of a slice/str-field slice, both stages

x.slicefield[:] / x.strfield[2:] emitted MOVQ $0 for the omitted hi
bound on BOTH stages (N_IDENT-gated dispatch; symmetric, so byte
identity never caught it) in all four sites: cgexpr N_SLICE + pushargs
(cstage), cgslice + pushargsrev (wwstage). The new arm re-evaluates
the pure field read for its {ptr,len,cap} header and takes .len,
covering local, viaptr, dot-chain, (*p), arr[i], and global inners.
Call inners still loud-reject upstream. Sibling of the #252/#257
array-field arms.

test/wcc/data/dotfield_slice_defhi/case.ww

@@ -0,0 +1,49 @@
+//ww:run
+// Default-hi slice of a slice/str FIELD (`x.f[:]`, `x.f[2:]`) —
+// expression position. The pre-fix default-hi dispatch was
+// N_IDENT-gated on BOTH stages, so an N_DOT base fell to MOVQ $0
+// (len 0 / negative, byteid-blind). Covers local, viaptr, dot-chain,
+// (*p) spelling, and struct-array-element inners.
+package main;
+
+type inner = struct {
+	buf: []i32,
+	name: str,
+};
+
+type outer = struct {
+	pad: i64,
+	in: inner,
+};
+
+export fn main() int = {
+	let arr: [4]i32 = [10, 20, 30, 40];
+	let v: inner = inner{ buf = arr[:], name = "hello" };
+	let b1: []i32 = v.buf[:];
+	if (len(b1) != 4) { return 1; };
+	let b2: []i32 = v.buf[1:];
+	if (len(b2) != 3) { return 2; };
+	if (b2[0] != 20) { return 3; };
+	let s1: str = v.name[2:];
+	if (len(s1) != 3) { return 4; };
+	let p: *inner = &v;
+	let b3: []i32 = p.buf[:];
+	if (len(b3) != 4) { return 5; };
+	let s2: str = p.name[1:];
+	if (len(s2) != 4) { return 6; };
+	let o: outer = outer{ pad = 7, in = inner{ buf = arr[:], name = "world" } };
+	let b4: []i32 = o.in.buf[:];
+	if (len(b4) != 4) { return 7; };
+	let b5: []i32 = o.in.buf[2:];
+	if (len(b5) != 2) { return 8; };
+	if (b5[1] != 40) { return 9; };
+	let s3: str = o.in.name[3:];
+	if (len(s3) != 2) { return 10; };
+	let b6: []i32 = (*p).buf[:];
+	if (len(b6) != 4) { return 11; };
+	let xs: [2]inner = [v, v];
+	let b7: []i32 = xs[1].buf[:];
+	if (len(b7) != 4) { return 12; };
+	if (b7[3] != 40) { return 13; };
+	return 0;
+};