etf: add MAP_EXT (tag 116) for Erlang/Elixir maps

Python dict round-trips as an Erlang/Elixir map. Golden vectors from
real Erlang term_to_binary verify wire compatibility for:
  - empty map (#{})
  - keyed map with binary keys + mixed values
  - {ok, #{status => ok, balance => N}} (Elixir gen_server reply shape)

122 tests green. Required for the unfeed ErlangDistTransport which
decodes Elixir wallet RPC replies.

erldistpy/etf.py

@@ -27,6 +27,7 @@ LIST_EXT = 108
 BINARY_EXT = 109
 SMALL_BIG_EXT = 110
 LARGE_BIG_EXT = 111
+MAP_EXT = 116
 NEW_PID_EXT = 88
 NEWER_REFERENCE_EXT = 90
 ATOM_UTF8_EXT = 118
@@ -90,6 +91,8 @@ def _encode(term: object) -> bytes:
         return _encode_tuple(term)
     if isinstance(term, list):
         return _encode_list(term)
+    if isinstance(term, dict):
+        return _encode_map(term)
     if isinstance(term, Pid):
         return _encode_pid(term)
     if isinstance(term, Reference):
@@ -134,6 +137,12 @@ def _encode_tuple(t: tuple) -> bytes:
     return out + b"".join(_encode(x) for x in t)
 
 
+def _encode_map(d: dict) -> bytes:
+    out = bytes([MAP_EXT]) + struct.pack(">I", len(d))
+    out += b"".join(_encode(k) + _encode(v) for k, v in d.items())
+    return out
+
+
 def _encode_list(lst: list) -> bytes:
     if not lst:
         return bytes([NIL_EXT])
@@ -249,6 +258,15 @@ def _decode(data: bytes, off: int) -> tuple[object, int]:
             # Improper list — uncommon in our protocol; surface it
             raise ETFError(f"improper list tail: {tail!r}")
         return items, off
+    if tag == MAP_EXT:
+        arity = struct.unpack(">I", data[off:off + 4])[0]
+        off += 4
+        result: dict = {}
+        for _ in range(arity):
+            k, off = _decode(data, off)
+            v, off = _decode(data, off)
+            result[k] = v
+        return result, off
     if tag == NEW_PID_EXT:
         node, off = _decode(data, off)
         if not isinstance(node, Atom):

tests/test_etf.py

@@ -49,6 +49,13 @@ GOLDEN = [
     ("nested",
      "83680364000867656e5f63616c6c6c0000000168026400046e6f646564000c77616c6c65744063616d6d796a680264000b6765745f62616c616e6365640003786d72",
      (Atom("gen_call"), [(Atom("node"), Atom("wallet@cammy"))], (Atom("get_balance"), Atom("xmr")))),
+    ("empty_map", "837400000000", {}),
+    ("addr_map",
+     "8374000000026d00000007616464726573736d00000005786d722d316d0000000863757272656e6379640003786d72",
+     {b"address": b"xmr-1", b"currency": Atom("xmr")}),
+    ("nested_ok",
+     "8368026400026f6b740000000264000762616c616e6365620012d6876400067374617475736400026f6b",
+     (Atom("ok"), {Atom("balance"): 1234567, Atom("status"): Atom("ok")})),
 ]
 
 
@@ -67,6 +74,8 @@ def test_decode_golden(name, hex_in, expected):
     (), (1,), (Atom("ok"), b"value"),
     [], [1, 2, 3], [Atom("a"), b"b", 3],
     (Atom("gen_call"), [], (Atom("ping"),)),
+    {}, {Atom("k"): 1}, {b"a": b"b", b"c": 42},
+    (Atom("ok"), {Atom("status"): Atom("ok"), Atom("count"): 7}),
 ])
 def test_round_trip(term):
     assert decode(encode(term)) == term