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>

plans/agent-briefings/conformance-loop.md

@@ -87,7 +87,7 @@ Blocked with specifics and move to the next candidate next iteration.
 
 - [x] common-lisp — migrated 487/487 (counters; driver extended for per-suite counters+preloads)
 - [x] erlang — migrated 761/761 (dict; pass/count → :failed = count-pass)
-- [ ] feed
+- [x] feed — migrated 189/189 (counters; test-harness.sx preload for counters+helper)
 - [ ] forth
 - [ ] go
 - [ ] js
@@ -100,6 +100,13 @@ Blocked with specifics and move to the next candidate next iteration.
 
 ## Progress log (newest first)
 
+- 2026-06-07 — feed: migrated to `MODE=counters`, 189/189 exact parity (basic 30,
+  fanout 29, rank 24, integration 22, content 15, notify 8, home 6, dedupe 9, trending 11,
+  mute 9, page 14, thread 12). Canonical counters shape: fresh session per suite, shared
+  preloads, single feed-test-pass/feed-test-fail pair. Lifted the old script's inline
+  epoch-2 counter+helper defs into lib/feed/test-harness.sx (preloaded last). No driver
+  change — only conformance.conf + test-harness.sx + shim. Kept historical scoreboard
+  schema (suites{name:{pass,fail}}, total_pass/total_fail/total).
 - 2026-06-07 — erlang: migrated to `MODE=dict`, 761/761 exact parity (tokenize 62,
   parse 52, eval 408, runtime 93, ring 4, ping-pong 4, bank 8, echo 7, fib 8, ffi 37,
   vm 78). Erlang exposes pass + *count* (total) counters, not pass/fail, so each suite's