Recover agent-loop progress: lua/prolog/forth/erlang/haskell phases 1-2

Salvaged from worktree-agent-* branches killed during sx-tree MCP outage:
- lua: tokenizer + parser + phase-2 transpile (~157 tests)
- prolog: tokenizer + parser + unification (72 tests, plan update lost to WIP)
- forth: phase-1 reader/interpreter + phase-2 colon/VARIABLE (134 tests)
- erlang: tokenizer + parser (114 tests)
- haskell: tokenizer + parse tests (43 tests)

Cherry-picked file contents only, not branch history, to avoid pulling in
unrelated ocaml-vm merge commits that were in those branches' bases.

lib/forth/interpreter.sx

@@ -0,0 +1,48 @@
+;; Forth interpreter loop — interpret mode only (Phase 1).
+;; Reads whitespace-delimited words, looks them up, executes.
+;; Numbers (parsed via BASE) push onto the data stack.
+;; Unknown words raise "?".
+
+(define
+  forth-execute-word
+  (fn (state word) (let ((body (get word "body"))) (body state))))
+
+(define
+  forth-interpret-token
+  (fn
+    (state tok)
+    (let
+      ((w (forth-lookup state tok)))
+      (if
+        (not (nil? w))
+        (forth-execute-word state w)
+        (let
+          ((n (forth-parse-number tok (get state "base"))))
+          (if
+            (not (nil? n))
+            (forth-push state n)
+            (forth-error state (str tok " ?"))))))))
+
+(define
+  forth-interpret
+  (fn
+    (state src)
+    (for-each
+      (fn (tok) (forth-interpret-token state tok))
+      (forth-tokens src))
+    state))
+
+;; Convenience: build a fresh state with primitives loaded.
+(define
+  forth-boot
+  (fn () (let ((s (forth-make-state))) (forth-install-primitives! s) s)))
+
+;; Run source on a fresh state and return (state, output, stack-top-to-bottom).
+(define
+  forth-run
+  (fn
+    (src)
+    (let
+      ((s (forth-boot)))
+      (forth-interpret s src)
+      (list s (get s "output") (reverse (get s "dstack"))))))