lib: split strings tests per ref/hare/strings families + helpers file

Unblocked by the package-mode ruling: same-package test files now
compose everywhere (coordinator dir-mode owns test-library and the
libbyteid dir entry owns byte identity), so the per-file standalone
constraint is gone and Go's stdlib layout applies directly. The
15-banner strings_test.ww monolith dissolves into per-ref-file
family files — dup, concat, trim, sub, utf8, iter, tokenize
(tokenize+splitn+cut, all of ref tokenize.ha), pad, replace — plus
helpers_test.ww holding exactly the one cross-family helper (streq),
Go's shared test-helper idiom. Pure moves; family-local helpers
(expect_str_token et al.) stay in their family file. Leading banners
that merely restated the filename dropped; 6 interior sub-family
boundaries remain. lib/ census 141 -> 132.

libbyteid gains the dx() dir-mode entry form decided in B2: compose
every *_test.ww under moddir exactly as pkgcombined does
(module-reset separators, byte-lex order) and byte-id the composed
root through both driver stages. The five per-file strings fx
entries retire into one dx("lib/strings") — every source line they
covered is inside the composed unit, and the completeness scan still
keys the module. NENTEXPECT 60 -> 56.

Recorded, not split: fmt_test's 13 banners are scenario groups over
the one fprint surface and ref/hare/fmt itself keeps a single
+test.ha — a print/wrappers split would fight the ref; memio stays
per the standing ruling (everything ww ports lives in ref
stream.ha).

lib/strings/utf8_test.ww

@@ -0,0 +1,23 @@
+// Vectors mirror ref/hare/strings/utf8.ha where ww can express them.
+// External strings_test package per the Go foo_test idiom (rule-5/9
+// carve-out, task #16); the shared streq helper lives in
+// helpers_test.ww — same-package test files compose in dir-mode
+// (the package, not the file, is the unit of testing).
+package strings_test;
+
+import strings;
+import encoding.utf8;
+
+// ref/hare/strings/utf8.ha:31. Validation-half coverage lives in
+// lib/encoding/utf8 (utf8test) per CLAUDE.md rule 9 carve-out.
+
+@test fn utf8_roundtrip_cases() void = {
+	let s: str = "hello";
+	let b: []u8 = strings.toutf8(s);
+	assert(!(b.len != 5));
+	assert(!(b[0] != 104u8));	// 'h'
+	let r: str = strings.frombytes(b);
+	assert(!(!streq(r, "hello")));
+	assert(!(r.ptr != s.ptr));	// borrowed, not copied
+};
+