plans: five rose-ash subsystem plans + three loop briefings

Plans for acl-on-sx (Datalog), flow-on-sx (Scheme), feed-on-sx (APL),
mod-on-sx (Prolog), search-on-sx (Haskell). Each is a 4-phase queue
sitting on its respective guest language, targeting rose-ash needs:
access control, durable workflows, activity feeds, moderation, search.
Federation extension in Phase 4 of each (plugs into fed-sx).

Briefings for the three loops we're kicking off now: acl-loop,
flow-loop, feed-loop. mod-sx and search-sx briefings will follow
once the first three have surfaced any shared infrastructure
worth extracting to lib/guest/.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plans/feed-on-sx.md

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+# feed-on-sx: Activity Feeds on APL
+
+Timelines, notifications, activity aggregation. The math is array math: filter, sort,
+reduce, scan, outer product. APL is the densest possible expression of feed
+composition — a fanout-and-rank pipeline reads as a single line.
+
+rose-ash needs: per-user home timeline, notification feed, activity stream digestion,
+backfill for new follows, deduplication across cross-posts. Every operation is an
+array-shaped transformation.
+
+End-state: an APL-flavored layer on `lib/apl/` with feed-specific combinators
+(`fanout`, `dedupe`, `score`, `rank`), an SX adapter for callers who don't want raw
+APL, ACL visibility filtering via `lib/acl/`, federation via fed-sx.
+
+## Status (rolling)
+
+`bash lib/feed/conformance.sh` → **0/0** (not yet started)
+
+## Ground rules
+
+- **Scope:** only touch `lib/feed/**` and `plans/feed-on-sx.md`. Do **not** edit
+  `spec/`, `hosts/`, `shared/`, `lib/apl/**`, or other `lib/<lang>/`. You may
+  **import** from `lib/apl/` (public API in `lib/apl/apl.sx`); do **not** modify APL.
+- **Shared-file issues** go under "Blockers" with a minimal repro; do not fix here.
+- **SX files:** use `sx-tree` MCP tools only.
+- **Architecture:** an activity is a small dict (`{:actor :verb :object :at :tags}`); a
+  stream is an APL vector of such dicts. Operations are APL primitives lifted onto
+  this shape. SX adapter exposes ergonomic API to non-APL callers.
+- **Unicode:** raw UTF-8 in `.sx` files. APL glyphs land directly.
+- **Commits:** one feature per commit. Keep Progress log updated and tick boxes.
+
+## Architecture sketch
+
+```
+Raw activities (any shape)              Per-user view
+        │                                    ▲
+        ▼                                    │
+lib/feed/normalize.sx              lib/feed/timeline.sx
+  — {:actor :verb :object             — (timeline user)
+     :at :tags} record                — applies filter ∘ rank ∘ take
+        │                                    ▲
+        ▼                                    │
+lib/feed/stream.sx                  lib/feed/rank.sx
+  — APL vector of activities          — velocity, recency
+  — filter, sort, take                — TF-IDF-ish over :tags
+        │                                    ▲
+        ▼                                    │
+lib/feed/fanout.sx                  lib/feed/dedupe.sx
+  — followers vector                  — group by :object
+  — activities ∘.× followers          — collapse cross-posts
+  — flatten + dedupe
+        │
+        ▼
+lib/feed/api.sx           lib/feed/fed.sx
+  — (feed/post activity)    — inbox via fed-sx
+  — (feed/timeline user)    — backfill on subscribe
+  — (feed/notify user)
+```
+
+## Phase 1 — Stream model + basic ops
+
+- [ ] `lib/feed/normalize.sx` — activity record schema; coerce arbitrary inputs
+- [ ] `lib/feed/stream.sx` — APL vector representation; filter by predicate; sort by
+  `:at`; take N (`↑`); reverse (`⌽`)
+- [ ] `lib/feed/api.sx` — `(feed/post activity)`, `(feed/all)`
+- [ ] `lib/feed/tests/basic.sx` — 15+ cases: post, query, filter, sort
+- [ ] `lib/feed/scoreboard.{json,md}`
+- [ ] `lib/feed/conformance.sh`
+
+## Phase 2 — Fanout via outer product
+
+- [ ] follower graph: `followers user → vector of user ids`
+- [ ] fanout: activities `∘.×` followers → matrix `(activity, follower)` pairs
+- [ ] flatten to inbox events vector
+- [ ] dedupe — group by `(actor, verb, object)` collapse to one inbox event per
+  receiver
+- [ ] `lib/feed/tests/fanout.sx` — 20+ cases: small graph, mutual follow, popular
+  actor (high-fanout), cross-post dedupe
+
+## Phase 3 — Aggregation + ranking
+
+- [ ] group-by — `(actor, day) → count` via key-reduce
+- [ ] velocity score — recent activity count over window
+- [ ] recency score — decay by age
+- [ ] composite rank — weighted sum of components
+- [ ] top-N per timeline
+- [ ] `lib/feed/tests/rank.sx` — 20+ cases: ranking stable on tie, decay shape,
+  per-user weighting
+
+## Phase 4 — Visibility filter + federation
+
+- [ ] ACL filter — each candidate activity passed through `(acl/permit? viewer :read
+  activity)`
+- [ ] fed-sx outbound — local `feed/post` fans out to remote followers' inboxes
+- [ ] fed-sx inbound — peer activities arrive at local inbox
+- [ ] backfill on subscribe — request peer history, merge into local stream
+- [ ] `lib/feed/tests/integration.sx` — federated timeline with ACL applied
+
+## Progress log
+
+(loop fills this in)
+
+## Blockers
+
+(loop fills this in)