selfhost+test: nodeisstr handles N_INDEX of [N]str (#34)

wwstage's nodeisstr (cgenutil) didn't recognize N_INDEX-of-[N]str.
cgindex emitted only the ptr-half MOVQ when the result was used as
a str arg (call, .len access, str streq), so the .len half read
stack residue. Surfaced by worker-21 during #21 dev — pre-#21
slotsize=24B masked the read-side defect; post-#21 (16B stride)
exposed it. cstage's typed-AST node_isstr handles this naturally;
wwstage's untyped pattern walks the base ident's tnode shape.

Added N_INDEX arm to nodeisstr: walk the indexed base's tnode
through N_TARRAY / N_TSLICE / N_TPTR.lhs, return isstrtype on the
element. Mirrors cgindex's own base-type walk byte-for-byte in
shape so the two now agree on load-shape decisions.

Not covered (separate bugs, separately filed):
- N_UN(TK_STAR) of *str — cgun itself never loads .len into BX.
- tuple `.1` of str — N_TTUPLE path has its own load shape.
- alias-typed base (`type a = [N]str`) — N_TNAME isn't peeled; cgindex
  doesn't peel it either, so agreement holds. Outside #34 scope.

Test 711: 3 new rows — barelet_index_call_arg (streq direct arg),
nested_call_index_arg (f(g(argv[i])) — nested-call recursion),
barelet_index_len_arg (sister regression-pin for cgindex element
stride in bare-let context; pins a different code path that was
already correct post-#21).

The pre-existing wwstage `..findflag(SB)` symbol-mangling bug in
getopttest wwstage build is filed as task #37, not in this commit's
scope.

test/wcc/711_arrlit_str_full.c

@@ -135,6 +135,74 @@ static const struct row rows[] = {
 	  "    return (a[0] + a[1] + a[2]): i32;\n"
 	  "};\n",
 	  6 },
+	/* 8. Bare-let [N]str index as a call arg (task #34). Pre-#34
+	 * wwstage emitted PUSHQ AX only for argv[i] — the str value's
+	 * BX=len half was dropped, and streq's `a.len` parameter read
+	 * stack residue. Symptom under getopttest: rc=11 pre-#21, then
+	 * 139 once #21 doubled the slot stride. Now byte-identical to
+	 * cstage at the call site. streq("files.txt",argv[2]) → 0 (eq)
+	 * → return 0. Pre-fix wwstage returned 11. */
+	{ "barelet_index_call_arg",
+	  "fn streq(a: str, b: str) bool = {\n"
+	  "    if (a.len != b.len) { return false; };\n"
+	  "    let i: i32 = 0;\n"
+	  "    for (i < a.len) {\n"
+	  "        if (a[i] != b[i]) { return false; };\n"
+	  "        i += 1;\n"
+	  "    };\n"
+	  "    return true;\n"
+	  "};\n"
+	  "fn main() i32 = {\n"
+	  "    let argv: [3]str;\n"
+	  "    argv[0] = \"ls\";\n"
+	  "    argv[1] = \"-Fahs\";\n"
+	  "    argv[2] = \"files.txt\";\n"
+	  "    if (!streq(argv[2], \"files.txt\")) { return 11; };\n"
+	  "    if (!streq(argv[1], \"-Fahs\")) { return 12; };\n"
+	  "    if (!streq(argv[0], \"ls\")) { return 13; };\n"
+	  "    return 0;\n"
+	  "};\n",
+	  0 },
+	/* 9. Nested call: `f(g(argv[i]))` exercises the full call-arg
+	 * packer with an N_INDEX-of-str inside an N_CALL inside another
+	 * N_CALL. Pins that the inner N_INDEX recognition rides through
+	 * the same pushargsrev path that the simple row uses. dup1 here
+	 * is identity-on-str so the outer streq receives g(argv[2]),
+	 * which must be 9 bytes ("files.txt"). */
+	{ "nested_call_index_arg",
+	  "fn dup1(s: str) str = { return s; };\n"
+	  "fn streq(a: str, b: str) bool = {\n"
+	  "    if (a.len != b.len) { return false; };\n"
+	  "    let i: i32 = 0;\n"
+	  "    for (i < a.len) {\n"
+	  "        if (a[i] != b[i]) { return false; };\n"
+	  "        i += 1;\n"
+	  "    };\n"
+	  "    return true;\n"
+	  "};\n"
+	  "fn main() i32 = {\n"
+	  "    let argv: [3]str;\n"
+	  "    argv[0] = \"a\";\n"
+	  "    argv[1] = \"bb\";\n"
+	  "    argv[2] = \"files.txt\";\n"
+	  "    if (!streq(dup1(argv[2]), \"files.txt\")) { return 11; };\n"
+	  "    return 0;\n"
+	  "};\n",
+	  0 },
+	/* 10. Index .len as a call arg via a one-arg consumer. Confirms
+	 * cgindex's (AX, BX) load still drives a single-i32 push when
+	 * the field selector picks the .len half off the str element.
+	 * Different code path from rows 8-9 (no str-arg push) but
+	 * uses the same N_INDEX base. 5 chars in "hello". */
+	{ "barelet_index_len_arg",
+	  "fn ident(n: i32) i32 = { return n; };\n"
+	  "fn main() i32 = {\n"
+	  "    let xs: [2]str;\n"
+	  "    xs[0] = \"hi\";\n"
+	  "    xs[1] = \"hello\";\n"
+	  "    return ident(xs[1].len: i32);\n"
+	  "};\n",
+	  5 },
 };
 
 static int