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/bin/sx_server.ml

@@ -353,6 +353,38 @@ let make_server_env () =
       (try env_get env name with _ -> Nil)
     | _ -> Nil);
 
+  (* Character classification — platform primitives for spec/parser.sx *)
+  bind "ident-start?" (fun args ->
+    match args with
+    | [String s] when String.length s = 1 ->
+      let c = s.[0] in
+      Bool (c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c = '_' || c = '~'
+            || c = '!' || c = '?' || c = '+' || c = '-' || c = '*' || c = '/'
+            || c = '=' || c = '<' || c = '>' || c = '&' || c = '|' || c = '%'
+            || c = '^' || c = '$' || c = '#')
+    | _ -> Bool false);
+  bind "ident-char?" (fun args ->
+    match args with
+    | [String s] when String.length s = 1 ->
+      let c = s.[0] in
+      Bool (c >= 'a' && c <= 'z' || c >= 'A' && c <= 'Z' || c = '_' || c = '~'
+            || c = '!' || c = '?' || c = '+' || c = '-' || c = '*' || c = '/'
+            || c = '=' || c = '<' || c = '>' || c = '&' || c = '|' || c = '%'
+            || c = '^' || c = '$' || c = '#'
+            || c >= '0' && c <= '9' || c = '.' || c = ':')
+    | _ -> Bool false);
+  bind "char-numeric?" (fun args ->
+    match args with
+    | [String s] when String.length s = 1 ->
+      Bool (s.[0] >= '0' && s.[0] <= '9')
+    | _ -> Bool false);
+  bind "parse-number" (fun args ->
+    match args with
+    | [String s] ->
+      (try Number (float_of_string s)
+       with _ -> Nil)
+    | _ -> Nil);
+
   bind "escape-string" (fun args ->
     match args with
     | [String s] ->

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)