test framework phase 1: forward decls, void-return fix, suite migration

Completes the Phase 1 runner and migrates the 11 test files onto it.

- forward-declare the registry accessors in the test preamble; they are
  defined at the end of the unit but the El runner is compiled in between
- eltest.el: explicit trailing return in the void emit_* helpers, which
  otherwise lower to 'return println(...)' and fail to compile
- test files import runtime/eltest.el explicitly, using the language's own
  textual import mechanism rather than compiler-side auto-injection
- DESIGN.md 6.5: gate on allocation COUNT AND BYTES, not count alone

Verified: self-hosting fixpoint byte-identical (gen2 == gen3). 6 of 11
suites run and report per-test timing. The other 5 fail to COMPILE, and
fail identically under the committed compiler -- pre-existing breakage
this framework makes visible for the first time.

lang/tests/native/test_string.el

@@ -1,3 +1,4 @@
+import "../../runtime/eltest.el"
 // test_string.el - native test suite for runtime/string.el
 //
 // Covers: type conversions, core primitives, comparison and search,