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/getopt/getopttest.ww

@@ -19,7 +19,7 @@
 // per task #15 — but the nested-if shape was the original workaround
 // and is preserved here as a regression marker.
 
-package getopt;
+package getopt_test;
 
 import getopt;
 import io;