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/conformance.conf

@@ -0,0 +1,82 @@
+# Feed-on-SX conformance config — sourced by lib/guest/conformance.sh.
+#
+# Every feed suite runs in a fresh session with the same preloads and a single
+# pass/fail counter pair — the canonical MODE=counters shape. The counters and
+# the feed-test helper (previously defined inline in the old conformance.sh) are
+# preloaded via lib/feed/test-harness.sx.
+
+LANG_NAME=feed
+MODE=counters
+COUNTERS_PASS=feed-test-pass
+COUNTERS_FAIL=feed-test-fail
+TIMEOUT_PER_SUITE=300
+
+PRELOADS=(
+  spec/stdlib.sx
+  lib/r7rs.sx
+  lib/apl/runtime.sx
+  lib/feed/normalize.sx
+  lib/feed/stream.sx
+  lib/feed/api.sx
+  lib/feed/fanout.sx
+  lib/feed/dedupe.sx
+  lib/feed/aggregate.sx
+  lib/feed/rank.sx
+  lib/feed/acl.sx
+  lib/feed/fed.sx
+  lib/feed/content.sx
+  lib/feed/notify.sx
+  lib/feed/home.sx
+  lib/feed/trending.sx
+  lib/feed/mute.sx
+  lib/feed/page.sx
+  lib/feed/thread.sx
+  lib/feed/test-harness.sx
+)
+
+SUITES=(
+  "basic:lib/feed/tests/basic.sx"
+  "fanout:lib/feed/tests/fanout.sx"
+  "rank:lib/feed/tests/rank.sx"
+  "integration:lib/feed/tests/integration.sx"
+  "content:lib/feed/tests/content.sx"
+  "notify:lib/feed/tests/notify.sx"
+  "home:lib/feed/tests/home.sx"
+  "dedupe:lib/feed/tests/dedupe.sx"
+  "trending:lib/feed/tests/trending.sx"
+  "mute:lib/feed/tests/mute.sx"
+  "page:lib/feed/tests/page.sx"
+  "thread:lib/feed/tests/thread.sx"
+)
+
+# Preserve the historical scoreboard schema so consumers of
+# lib/feed/scoreboard.json keep working.
+emit_scoreboard_json() {
+  local n=${#GC_NAMES[@]} i
+  printf '{\n'
+  printf '  "suites": {\n'
+  for ((i=0; i<n; i++)); do
+    [ "$i" -gt 0 ] && printf ',\n'
+    printf '    "%s": {"pass": %d, "fail": %d}' \
+      "${GC_NAMES[$i]}" "${GC_PASS[$i]}" "${GC_FAIL[$i]}"
+  done
+  printf '\n  },\n'
+  printf '  "total_pass": %d,\n' "$GC_TOTAL_PASS"
+  printf '  "total_fail": %d,\n' "$GC_TOTAL_FAIL"
+  printf '  "total": %d\n' "$GC_TOTAL"
+  printf '}\n'
+}
+
+emit_scoreboard_md() {
+  local n=${#GC_NAMES[@]} i p f
+  printf '# feed Conformance Scoreboard\n\n'
+  printf '_Generated by `lib/feed/conformance.sh`_\n\n'
+  printf '| Suite | Pass | Fail | Total |\n'
+  printf '|-------|-----:|-----:|------:|\n'
+  for ((i=0; i<n; i++)); do
+    p=${GC_PASS[$i]}; f=${GC_FAIL[$i]}
+    printf '| %s | %d | %d | %d |\n' "${GC_NAMES[$i]}" "$p" "$f" "$((p+f))"
+  done
+  printf '| **Total** | **%d** | **%d** | **%d** |\n' \
+    "$GC_TOTAL_PASS" "$GC_TOTAL_FAIL" "$GC_TOTAL"
+}