GUEST: step 4 — lib/guest/pratt.sx operator-table format + lookup

Extracted the data-half of Pratt-style precedence parsing: the operator
table format and lookup. The climbing loop stays per-language because
the two canaries use opposite conventions (lua: higher prec = tighter;
prolog: lower prec = tighter, with xfx/xfy/yfx assoc tags) — forcing
one shared loop adds callback indirection that obscures more than it
shares. The brief's literal ask is "Grammar is a dict, not hardcoded
cond" and that's what gets shared.

Entry shape: (NAME PREC ASSOC). Three accessors: pratt-op-name /
pratt-op-prec / pratt-op-assoc. One traversal: pratt-op-lookup.

Ported lua/parser.sx — replaced 18-clause cond and the
lua-binop-right? hardcoded `or` with a 15-entry lua-op-table, now
queried via pratt-op-lookup. Ported prolog/parser.sx — pl-op-find
(linear walk reimpl) deleted; pl-op-lookup wraps pratt-op-lookup;
pl-token-op simplified to return the entry directly.

Verification:
- lua/test.sh: 185/185 = baseline.
- prolog/conformance.sh: 590/590 = baseline (timestamp-only diff).

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

lib/guest/pratt.sx

@@ -0,0 +1,28 @@
+;; lib/guest/pratt.sx — operator-table format + lookup for Pratt-style
+;; precedence climbing.
+;;
+;; The climbing loop stays per-language because the two canaries use
+;; opposite conventions (Lua: higher prec = tighter; Prolog: lower prec =
+;; tighter, with xfx/xfy/yfx assoc tags). Forcing a single loop adds
+;; callback indirection that obscures more than it shares.
+;;
+;; What IS shared and gets extracted: the operator-table format and lookup.
+;; "Grammar is a dict, not hardcoded cond."
+;;
+;; Entry shape: (NAME PREC ASSOC).
+;;   NAME   — string, the operator's source token.
+;;   PREC   — integer, in the host's own convention.
+;;   ASSOC  — :left | :right | :none for languages with traditional
+;;            associativity, or "xfx" / "xfy" / "yfx" for Prolog-style.
+
+(define
+  pratt-op-lookup
+  (fn (table name)
+    (cond
+      ((empty? table) nil)
+      ((= (first (first table)) name) (first table))
+      (:else (pratt-op-lookup (rest table) name)))))
+
+(define pratt-op-name  (fn (entry) (first entry)))
+(define pratt-op-prec  (fn (entry) (nth entry 1)))
+(define pratt-op-assoc (fn (entry) (nth entry 2)))