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/runtime/eltest.el

@@ -130,6 +130,7 @@ fn _elt_emit_result(json_mode: Bool, name: String, fails: Int, ns: Int, asserts:
         return
     }
     println("ok    " + name + "  (" + _elt_ms(ns) + "ms)")
+    return
 }
 
 fn _elt_emit_summary(json_mode: Bool, total: Int, failed: Int, ns: Int, asserts: Int) {
@@ -149,6 +150,7 @@ fn _elt_emit_summary(json_mode: Bool, total: Int, failed: Int, ns: Int, asserts:
     println(int_to_str(total) + " tests, " + int_to_str(passed) + " passed, "
         + int_to_str(failed) + " failed, " + int_to_str(asserts) + " assertions in "
         + _elt_ms(ns) + "ms")
+    return
 }
 
 // ── The runner ───────────────────────────────────────────────────────────────