fix(echo): patch two exhaustiveness gaps + extend TG-4 round-trip corpus

Two warnings-as-errors surfaced when compiling the echo/product additions
for the first time on a real OCaml toolchain:

  compiler/lib/typecheck.ml — `strand_type_of_ty` had no arms for TProd/TEcho.
  compiler/bin/main.ml     — the debug token printer had no arms for the 8 new
                             echo keyword tokens (ECHOCLOSE LOWER RESIDUE PAIR
                             FST SND ECHOADD ECHOEQ).

Both are straightforward exhaustiveness gaps; the design is preserved.
Both get StrandDefault / print_string "<NAME>" mirroring the nearest
neighbouring cases.

Also extend the TG-4 round-trip corpus in test_roundtrip.ml with one entry
per new echo constructor (echoClose, lower, residue, pair, fst, snd, echoAdd,
echoEq) so the parse/pretty/parse round-trip guarantee is explicitly exercised
for every form added by this PR.

Before: dune build fails; 532 tests (not reached)
After:  dune build succeeds; 548/548 tests pass (16 new TG-4 entries)

Author: hyperpolymath

compiler/bin/main.ml

@@ -167,7 +167,15 @@ let dump_tokens (filename : string) : unit =
      | STRING s    -> Printf.printf "STRING(%S)" s
      | IDENT s     -> Printf.printf "IDENT(%s)" s
      | GENERATOR n -> Printf.printf "GENERATOR(%d)" n
-     | EOF         -> print_string "EOF");
+     | EOF         -> print_string "EOF"
+     | ECHOCLOSE   -> print_string "ECHOCLOSE"
+     | LOWER       -> print_string "LOWER"
+     | RESIDUE     -> print_string "RESIDUE"
+     | PAIR        -> print_string "PAIR"
+     | FST         -> print_string "FST"
+     | SND         -> print_string "SND"
+     | ECHOADD     -> print_string "ECHOADD"
+     | ECHOEQ      -> print_string "ECHOEQ");
     print_newline ();
     if tok <> EOF then loop ()
   in

compiler/lib/typecheck.ml

@@ -584,6 +584,8 @@ and strand_type_of_ty (t : ty) : strand_type =
   | TBool -> StrandNamed "Bool"
   | TWord _ -> StrandDefault
   | TTangle _ -> StrandDefault
+  | TProd _ -> StrandDefault
+  | TEcho _ -> StrandDefault
 
 (** Convert a strand_type to a boundary element for self-crossing. *)
 and strand_to_type (st : strand_type) : strand_type = st

compiler/test/test_roundtrip.ml

@@ -100,6 +100,15 @@ let basic_corpus : (string * string) list = [
   ("nullary def",        "def x = identity");
   ("unary def",          "def f(a) = a");
   ("binary def",         "def f(a, b) = a + b");
+  (* Echo / structured-loss forms (PR #45) *)
+  ("echoClose",          "def x = echoClose(a)");
+  ("lower",              "def x = lower(a)");
+  ("residue",            "def x = residue(a)");
+  ("pair",               "def x = pair(a, b)");
+  ("fst",                "def x = fst(a)");
+  ("snd",                "def x = snd(a)");
+  ("echoAdd",            "def x = echoAdd(a, b)");
+  ("echoEq",             "def x = echoEq(a, b)");
 ]
 
 (* --------------------------------------------------------------------- *)