SX spec introspection: the spec examines itself via sx-parse

spec-introspect.sx: pure SX functions that read, parse, and analyze
spec files. No Python. The spec IS data — a macro transforms it into
explorer UI components.

- spec-explore: reads spec file via IO, parses with sx-parse, extracts
  sections/defines/effects/params, produces explorer data dict
- spec-form-name/kind/effects/params/source: individual extractors
- spec-group-sections: groups defines into sections
- spec-compute-stats: aggregate effect/define counts

OCaml kernel fixes:
- nth handles strings (character indexing for parser)
- ident-start?, ident-char?, char-numeric?, parse-number: platform
  primitives needed by spec/parser.sx when loaded at runtime
- _find_spec_file: searches spec/, web/, shared/sx/ref/ for spec files

83/84 Playwright tests pass. The 1 failure is client-side re-rendering
of the spec explorer (the client evaluates defpage content which calls
find-spec — unavailable on the client).

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

hosts/ocaml/lib/sx_primitives.ml

@@ -328,7 +328,11 @@ let () =
     match args with
     | [List l; Number n] | [ListRef { contents = l }; Number n] ->
       (try List.nth l (int_of_float n) with _ -> Nil)
-    | _ -> raise (Eval_error "nth: list and number"));
+    | [String s; Number n] ->
+      let i = int_of_float n in
+      if i >= 0 && i < String.length s then String (String.make 1 s.[i])
+      else Nil
+    | _ -> raise (Eval_error "nth: list/string and number"));
   register "cons" (fun args ->
     match args with
     | [x; List l] | [x; ListRef { contents = l }] -> List (x :: l)