Add _hs-custom-conversions dict and _hs-dynamic-converters list to
runtime.sx. hs-set-conversion!/hs-clear-conversion!/hs-add-dynamic-converter!/
hs-pop-dynamic-converter!/hs-clear-converters! helpers expose the API.
hs-coerce fallback now checks static dict then dynamic resolvers before
returning value unchanged.
Hand-roll MANUAL_TEST_BODIES for "can accept custom conversions" and
"can accept custom dynamic conversions" — previously SKIP (untranslated).
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>