lib/test: Go external-test packages (<mod>_test); retire decimaltest (#16 PREP-a)

30 lib test files: package <mod> -> <mod>_test, the sanctioned Go-over-Hare
departure (CLAUDE.md rule-9 carve-out; white-box testing deferred, not
forbidden). decimaltest retired per .ai/rob-16-decimaltest-ruling.md: Hare
ships no decimal_test.ha (engine covered transitively); coverage migrated
losslessly into stoftest rows (all-9s carry, nd>19 pure-decimal) + ftostest
f64_roundtrip mirroring ref/hare strconv ftos_test.ha tcsf64; k=0 micro-gap
documented at site. regex_test keeps its white-box internal probes under a
documented rule-7 divergence (rehome = task #9). 922_decimal_run + its 989
byte-id row removed with the fixture.

lib/strings/stringstest.ww

@@ -6,7 +6,11 @@
 // Vectors mirror ref/hare/strings/{dup,concat,trim,contains,index,
 // suffix,compare}.ha where ww can express them.
 
-package strings;
+// `_test` suffix: Go external-test-package idiom (sanctioned Go-over-Hare
+// departure for lib/ tests) — self-import is hard-rejected, so the test
+// lives in package strings_test and pulls the real package via import.
+// See CLAUDE.md rule-5/9 carve-out; task #16.
+package strings_test;
 
 import strings;
 import encoding.utf8;