cgen: sub-slice cap = base_cap - lo

A sub-slice `base[lo:hi]` now sets cap to base_cap - lo (the storage
remaining to the underlying end; Go/Hare-identical) instead of hi - lo
(== len). base_cap is the array length N for [N]T, or the .capacity
word carried in a slice/str header at +16. Authored once per stage in
the cg_base_cap / cgbasecap helper, applied at both cap sites: the
N_SLICE value path (which serves let-init since the prior commit) and
the call-arg push. Both stages stay byte-identical (find-4 closed).

cap arithmetic per ref/harec/src/eval.c:1017 (slice: slice.cap -=
start) and eval.c:1024 (array: cap = array.length - start); capacity
is a distinct field per ref/hare/rt/ensure.ha:4-8 and cap >= len per
ref/harec/src/check.c:596. Only the cap arithmetic transfers: the ptr
stays unscaled (lo*esz is #76) and eval.c's stricter start>=end bound
is not ported (ww's runtime bound is start>end).

str[lo:hi] yields str with a real .capacity (D1), so the str base uses
the same +16 load -- no downgrade to []u8. base_cap falls back to len
(prior behavior) where it isn't cleanly available: a non-ident base
(its header cap was discarded by cgexpr; len is likewise wrong for a
defaulted hi there, pre-existing) and a global str base (wwstage
cgslice has no global-str load, #73 -- the carve-out keeps both
stages byte-identical).

Test: 942_subslice_cap_run, table-driven over both drivers, array /
slice / str base + an append-no-realloc row, each shape chosen so
base_cap-lo != hi-lo.

Fold in three pre-existing fixtures that asserted the old cap == len
and so failed under the corrected semantics (project #20):
681_arr_elem_field_write (slice_field_value_write,
slice_field_ptr_write, slice_field_distinct_bytes),
693_dot_tagged_source (local_struct_slice_variant,
via_ptr_slice_variant, letinit_slice_roundtrip, top_level_global_slice),
and 695_match_bind_struct (slice_neg_control). Each cap word updated to
base_cap - lo: a [8]u8 base sliced at lo=0 yields cap 8 (5->8, 3->8);
distinct_bytes slices a [16]u8 at lo=0, yielding cap 16 (6->16). len /
mark / ptr assertions are unchanged -- only the cap word moved.

selfhost/cmd/wcc/cgenutil.ww

@@ -389,7 +389,13 @@ fn pushargsrev(c: *cgen, arg: *node, param: *node) i32 = {
 		emitline("\tMOVQ\tBX, DX\n");       // DX = hi
 		emitline("\tSUBQ\tAX, DX\n");       // DX = hi - lo = len
 		emitline("\tADDQ\tAX, CX\n");       // CX = base + lo = ptr
-		emitline("\tPUSHQ\tDX\n");          // cap
+		// cap = base_cap - lo (#20); AX=lo, BX free.
+		if (cgbasecap(c, base, "BX")) {
+			emitline("\tSUBQ\tAX, BX\n");
+			emitline("\tPUSHQ\tBX\n");  // cap
+		} else {
+			emitline("\tPUSHQ\tDX\n");  // cap = len
+		};
 		emitline("\tPUSHQ\tDX\n");          // len
 		emitline("\tPUSHQ\tCX\n");          // ptr (top)
 		return rest + 3;