Step 14: source locations — pos-to-loc, error-loc, sx-parse-loc — 15 tests

Pure SX layer: pos-to-loc (offset→line/col), error-loc (parse result→loc),
format-parse-error (human-readable error with source context line).
OCaml platform: cst_to_ast_loc (CST spans→loc dicts), sx-parse-loc
primitive (parse with locations), source-loc accessor.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/parser-combinators.sx

@@ -426,6 +426,47 @@
 
 ;; ── Layer 8: SX tokenizer ─────────────────────────────────────────
 
+(define
+  pos-to-loc
+  (fn
+    (input pos)
+    (let
+      loop
+      ((i 0) (line 1) (col 0))
+      (if
+        (>= i pos)
+        {:line line :col col}
+        (if
+          (= (char-at input i) "\n")
+          (loop (+ i 1) (+ line 1) 0)
+          (loop (+ i 1) line (+ col 1)))))))
+
+(define
+  error-loc
+  (fn (result input) (pos-to-loc input (result-pos result))))
+
+(define
+  format-parse-error
+  (fn
+    (result input)
+    (let
+      ((loc (error-loc result input))
+        (line-num (get loc :line))
+        (col-num (get loc :col))
+        (expected (result-expected result))
+        (lines (split input "\n"))
+        (source-line
+          (if (<= line-num (len lines)) (nth lines (- line-num 1)) "")))
+      (str
+        "line "
+        line-num
+        ", col "
+        col-num
+        ": expected "
+        expected
+        "\n"
+        source-line))))
+
 (define
   sx-comment
   (fn