conformance: migrate feed onto shared driver (counters, 189/189 parity)

Feed is the canonical MODE=counters shape: each suite runs in a fresh session
with shared preloads and a single feed-test-pass/feed-test-fail pair. Lifted the
old script's inline epoch-2 counter + feed-test helper defs into
lib/feed/test-harness.sx (preloaded last) so the driver can load them before
each suite. conformance.conf + 3-line shim; historical scoreboard schema
preserved. No driver change needed.

Parity verified 189/189 (0 fail), every suite matching baseline.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

lib/feed/test-harness.sx

@@ -0,0 +1,14 @@
+;; lib/feed/test-harness.sx — counter definitions for the feed conformance
+;; suites, lifted from the inline epoch-2 defs in the old conformance.sh so the
+;; shared driver (MODE=counters) can preload them before each suite.
+
+(define feed-test-pass 0)
+(define feed-test-fail 0)
+(define
+  feed-test
+  (fn
+    (name got expected)
+    (if
+      (= got expected)
+      (set! feed-test-pass (+ feed-test-pass 1))
+      (set! feed-test-fail (+ feed-test-fail 1)))))