Align OCaml parser with spec/parser.sx character classification

Replace permissive is_symbol_char (negative check — everything not a
delimiter) with spec-compliant is_ident_start/is_ident_char (positive
check matching the exact character sets documented in parser.sx).

Changes:
- ident-start: remove extra chars (|, %, ^, $) not in spec
- ident-char: add comma (,) per spec
- Comma (,) now handled as dedicated unquote case in match, not in
  the catch-all fallback — matches spec dispatch order
- Remove ~@ splice-unquote alias (spec only defines ,@)

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

hosts/ocaml/lib/sx_parser.ml

@@ -30,10 +30,21 @@ let skip_whitespace_and_comments s =
     | _ -> ()
   in go ()
 
-let is_symbol_char = function
-  | '(' | ')' | '[' | ']' | '{' | '}' | '"' | '\'' | '`'
-  | ' ' | '\t' | '\n' | '\r' | ',' | ';' -> false
-  | _ -> true
+(* Character classification — matches spec/parser.sx ident-start/ident-char.
+   ident-start: a-z A-Z _ ~ * + - > < = / ! ? &
+   ident-char:  ident-start plus 0-9 . : / # , *)
+let is_ident_start = function
+  | 'a'..'z' | 'A'..'Z' | '_' | '~' | '*' | '+' | '-'
+  | '>' | '<' | '=' | '/' | '!' | '?' | '&' -> true
+  | _ -> false
+
+let is_ident_char = function
+  | c when is_ident_start c -> true
+  | '0'..'9' | '.' | ':' | '#' | ',' -> true
+  | _ -> false
+
+(* Symbol reading uses ident_char; first char must be ident_start or digit/colon *)
+let is_symbol_char = is_ident_char
 
 let read_string s =
   (* s.pos is on the opening quote *)
@@ -116,20 +127,16 @@ let rec read_value s : value =
         go ()
       end
     in go ()
-  | '~' when s.pos + 1 < s.len && s.src.[s.pos + 1] = '@' ->
-    advance s; advance s; (* skip ~@ *)
-    List [Symbol "splice-unquote"; read_value s]
-  | _ ->
-    (* Check for unquote: , followed by non-whitespace *)
-    if s.src.[s.pos] = ',' && s.pos + 1 < s.len &&
-       s.src.[s.pos + 1] <> ' ' && s.src.[s.pos + 1] <> '\n' then begin
+  | ',' ->
+    (* Unquote / splice-unquote — matches spec: , always triggers unquote *)
     advance s;
     if s.pos < s.len && s.src.[s.pos] = '@' then begin
       advance s;
       List [Symbol "splice-unquote"; read_value s]
     end else
       List [Symbol "unquote"; read_value s]
-    end else begin
+  | _ ->
+    begin
       (* Symbol, keyword, number, or boolean *)
       let token = read_symbol s in
       if token = "" then raise (Parse_error ("Unexpected char: " ^ String.make 1 s.src.[s.pos]));