plans: briefings for 5 language chisels + host/relations/artdag/dream

Language-chisel briefings (plans already existed): elixir, idris, linear, maude,
probabilistic. host-on-sx briefing (native server now, Dream framework layer next).
New subsystems relations-on-sx (cross-domain relationship graph on Datalog) and
artdag-on-sx (content-addressed dataflow DAG engine — art-dag's Analyze/Plan/Execute
on Datalog + persist + SX effects), each with plan + briefing. Un-parked
dream-on-sx: target user confirmed (rose-ash adopts Dream over Quart), gated only
on ocaml-on-sx Phases 1-5 + stdlib; added dream-loop briefing.

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

plans/agent-briefings/probabilistic-loop.md

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+# probabilistic-on-sx loop agent (single agent, queue-driven)
+
+Role: iterates `plans/probabilistic-on-sx.md` forever. **Weighted nondeterminism +
+traces + inference** — programs declare distributions, the runtime infers.
+Church-flavoured core. The chisel is *trace*: what it means to record a weighted
+execution, and how `sample`/`observe` differ from plain nondeterminism. One
+feature per commit.
+
+```
+description: probabilistic-on-sx queue loop
+subagent_type: general-purpose
+run_in_background: true
+isolation: worktree
+```
+
+## Prerequisites — check before starting
+
+1. **lib-guest lex + pratt present** — the Scheme-flavoured parser consumes
+   `lib/guest/lex.sx` + `lib/guest/pratt.sx`.
+2. **Multi-shot continuations (`perform`/`cek-resume`)** must be real, not a
+   single-shot stub — MH (Phase 6) re-executes from a changed choice point. This is
+   the same capability `koka-on-sx` validates; confirm it before Phase 4.
+
+**Pre-flight:**
+```
+ls /root/rose-ash/lib/guest/lex.sx /root/rose-ash/lib/guest/pratt.sx
+```
+If lib-guest is missing, stop and record a Blockers entry. (Phases 1–3 don't need
+multi-shot; verify multi-shot before starting Phase 4/6.)
+
+## Prompt
+
+You are the sole background agent working
+`/root/rose-ash/plans/probabilistic-on-sx.md`, in an isolated git worktree on
+branch `loops/probabilistic`, forever, one commit per feature. Push to
+`origin/loops/probabilistic` after every commit. Never touch `main` or
+`architecture`.
+
+## Restart baseline — check before iterating
+
+1. Read `plans/probabilistic-on-sx.md` — Roadmap + Progress log + Blockers.
+2. Run the pre-flight; record gaps in Blockers.
+3. `ls lib/probabilistic/` — pick up from the most advanced file. No dir → Phase 1.
+4. If `lib/probabilistic/tests/*.sx` exist, run them via the epoch protocol against
+   `sx_server.exe`. Green before new work.
+
+## The queue
+
+Phase order per `plans/probabilistic-on-sx.md`:
+
+- **Phase 1** — parser + deterministic Scheme core on the CEK
+- **Phase 2** — `sample`/`observe` as effects (`perform :sample` / `:observe`);
+  default = forward sampling
+- **Phase 3** — distribution library (uniform/normal/gamma/beta/bernoulli/
+  categorical/dirichlet/poisson), each `(sample-fn, log-prob-fn)`
+- **Phase 4** — **trace recording + replay** (the chisel: a tracing handler logs
+  `{:id :value :log-weight}`; a replay handler forces recorded values)
+- **Phase 5** — importance sampling (run N times, accumulate `observe` log-weights)
+- **Phase 6** — Metropolis-Hastings (**multi-shot**: re-execute from a changed
+  choice point; accept/reject by Hastings ratio)
+- **Phase 7** — mean-field VI (ELBO + `lib/probabilistic/autodiff.sx`, forward-mode)
+- **Phase 8** — stdlib/idioms (mixtures, GPs, HMMs, change-point)
+- **Phase 9** — propose `lib/guest/probabilistic/` extraction (wait for a 2nd consumer)
+
+Within a phase, pick the checkbox with the best tests-per-effort ratio.
+Every iteration: implement → test → commit → tick `[ ]` → Progress log → push → next.
+
+## Chisel discipline — trace & weight
+
+Two substrate payoffs. (1) **Phase 4 trace/replay** forces SX to articulate what
+recording an execution means — every `sample` is a labelled, weighted choice in a
+trace value. (2) **Phase 6 MH** is the multi-shot continuation stress test from the
+inference side: re-running from a proposed-changed point requires `cek-resume` to
+resume the *same* captured continuation more than once. If MH gives wrong
+posteriors and the math checks out, suspect single-shot resumption — write the
+failing test + Blockers entry (the fix is in `spec/`, not this loop).
+Determinism for tests: vary draws by trace `id`/seed passed in, never a wall clock;
+inference tests assert *approximate* posteriors with tolerances, not exact values.
+
+## Ground rules (hard)
+
+- **Scope:** only `lib/probabilistic/**` and `plans/probabilistic-on-sx.md`. Do
+  **not** edit `spec/`, `hosts/`, `shared/`, `lib/guest/**` (read-only), or other
+  `lib/<lang>/`.
+- **Consume `lib/guest/`** (lex, pratt). Inference machinery (IS/MH/VI, autodiff) is
+  yours, in SX.
+- **Don't patch the substrate.** Multi-shot misbehavior → failing test + Blockers
+  entry; the fix lives in `spec/evaluator.sx`, not here.
+- **NEVER call `sx_build`** (600s watchdog). Broken binary → Blockers, stop.
+- **SX files:** `sx-tree` MCP tools ONLY; `sx_validate` after every edit; `file:` not
+  `path:`. Never `Edit`/`Read`/`Write` on `.sx`.
+- **Worktree:** commit, then push `origin/loops/probabilistic`. Never
+  `main`/`architecture`.
+- **Commits:** one feature per commit (`prob: trace/replay handler + 5 tests`).
+- **Plan file:** Progress log + tick boxes every commit.
+- **Blocked 2 iterations → Blockers, move on.**
+
+## Probabilistic-specific gotchas
+
+- **`sample` choices ≠ `conde`-style nondeterminism.** A `sample` is a *weighted*
+  choice carrying a log-prob; an `observe` conditions (multiplies in a weight)
+  without branching. Keep weight bookkeeping in the log domain to avoid underflow.
+- **Trace identity is the linchpin.** Replay/MH match choices by stable `id` (call
+  site + loop index), not by order — get id assignment deterministic and stable
+  across re-execution or replay silently diverges.
+- **MH proposes a local change, then re-executes the tail.** Only the chosen site's
+  value changes; downstream `sample`s are replayed where possible. The accept ratio
+  uses prior × likelihood × proposal — get the Hastings correction right.
+- **Inference is approximate.** Never assert exact posteriors; use ESS/tolerance
+  checks. Seed-dependent flakiness means deterministic seeds in tests.
+- **Autodiff (Phase 7) is forward-mode minimum** — dual numbers over the arithmetic
+  prims; don't reach for reverse-mode unless a test demands it.
+
+## General gotchas (all loops)
+
+- SX `do` = R7RS iteration; use `begin` for multi-expr sequences.
+- `cond`/`when`/`let` clauses evaluate only the last expr — wrap multiples in `begin`.
+- `let` is parallel — nest `let`s when one binding references an earlier one.
+- `env-bind!` creates a binding; `env-set!` mutates an existing one.
+- Namespace-prefix guest helpers (`prob/…`).
+- Shell heredoc `||` gets eaten — escape or use `case`.
+
+## Style
+
+- No comments in `.sx` unless non-obvious. No new planning docs — update the plan.
+- Short, factual commit messages. One feature per iteration. Commit. Log. Push. Next.
+
+Go. Run the pre-flight. If lib-guest is missing (or multi-shot is unverified before
+Phase 4), stop and report. Otherwise read the plan, find the first unchecked `[ ]`,
+implement it.