feat(relay): experimental CapabilityDescriptor schema

Frozen, JSON-serializable handshake payload the connector hands the future
RelayAdapter: char limit, draft-streaming/edit/threading flags, markdown
dialect, len_unit. Mostly a wire projection of PlatformEntry + the adapter
capability methods. contract_version gates additive-only evolution; declared
EXPERIMENTAL until >=2 Class-1 platforms validate it. from_json ignores
unknown keys (forward-compat) and fills optional defaults.

Phase 0, Task 0.2 of the gateway-relay plan.

gateway/relay/__init__.py

@@ -0,0 +1,11 @@
+"""Relay/connector support package for the Hermes gateway.
+
+EXPERIMENTAL. This package implements the gateway side of the "Gateway Gateway"
+relay design: a generic ``RelayAdapter`` plus the wire-serializable
+``CapabilityDescriptor`` the connector hands it at handshake time. The public
+API (module names, descriptor field set, transport protocol) MAY CHANGE without
+a deprecation cycle until at least two real Class-1 platforms (Discord +
+Telegram) have shaken out the schema.
+
+See ``docs/relay-connector-contract.md`` for the formal cross-repo interface.
+"""

gateway/relay/descriptor.py

@@ -0,0 +1,77 @@
+"""CapabilityDescriptor — the relay handshake payload. EXPERIMENTAL.
+
+The connector hands a ``CapabilityDescriptor`` to the gateway's ``RelayAdapter``
+at handshake time; it tells the adapter which platform it is fronting and which
+capabilities to advertise to the ``GatewayStreamConsumer`` (char limit,
+draft-streaming, edit/threading support, markdown dialect, length unit). It is
+the linchpin of the generalization: one gateway adapter serves Discord,
+Telegram, Matrix, Signal, ... without per-platform branching.
+
+EXPERIMENTAL: this schema MAY CHANGE without a deprecation cycle until at least
+two real Class-1 platforms have validated it. Evolution during the experimental
+phase is additive-only, gated by ``contract_version`` (see
+docs/relay-connector-contract.md).
+
+Field origins (most are a wire-serializable projection of ``PlatformEntry`` plus
+the per-instance capability methods on ``BasePlatformAdapter``):
+
+- ``max_message_length`` -> ``PlatformEntry.max_message_length`` / adapter
+  ``MAX_MESSAGE_LENGTH`` attribute (read by stream_consumer).
+- ``len_unit``           -> selects which ``message_len_fn`` the adapter installs
+  ("chars" = builtin len; "utf16" = Telegram-style UTF-16 code-unit counting).
+- ``supports_draft_streaming`` -> adapter ``supports_draft_streaming()`` probe.
+- ``supports_edit``      -> whether edit-based streaming is possible (Discord/
+  Telegram yes; Signal/SMS no -> consumer degrades to one-message-per-segment).
+- ``supports_threads``   -> ``create_handoff_thread`` capability flag.
+- ``markdown_dialect``   -> presentation hint (e.g. "markdown_v2", "discord").
+- ``emoji`` / ``platform_hint`` / ``pii_safe`` -> ``PlatformEntry`` fields of the
+  same name.
+"""
+
+from __future__ import annotations
+
+import json
+from dataclasses import asdict, dataclass
+
+# Bump additively (never reinterpret an existing field) during the experimental
+# phase; a breaking change requires updating both repos in lockstep.
+CONTRACT_VERSION = 1
+
+
+@dataclass(frozen=True)
+class CapabilityDescriptor:
+    """Immutable capability descriptor negotiated at relay handshake.
+
+    Frozen so a descriptor cannot be mutated after handshake — the adapter
+    advertises a fixed capability profile for the life of the connection.
+    """
+
+    contract_version: int
+    platform: str
+    label: str
+    max_message_length: int
+    supports_draft_streaming: bool
+    supports_edit: bool
+    supports_threads: bool
+    markdown_dialect: str
+    len_unit: str  # "chars" | "utf16"
+    emoji: str = "\U0001f50c"  # 🔌 default (matches PlatformEntry default)
+    platform_hint: str = ""
+    pii_safe: bool = False
+
+    def to_json(self) -> str:
+        """Serialize to a compact, stable JSON string for the handshake frame."""
+        return json.dumps(asdict(self), sort_keys=True, ensure_ascii=False)
+
+    @classmethod
+    def from_json(cls, data: str) -> "CapabilityDescriptor":
+        """Deserialize from a handshake JSON string.
+
+        Unknown keys are ignored (forward-compat: a newer connector may send
+        fields this gateway does not know yet); missing optional keys fall back
+        to dataclass defaults.
+        """
+        raw = json.loads(data)
+        known = {f for f in cls.__dataclass_fields__}  # type: ignore[attr-defined]
+        filtered = {k: v for k, v in raw.items() if k in known}
+        return cls(**filtered)

tests/gateway/relay/test_descriptor.py

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+"""Tests for the experimental CapabilityDescriptor (relay Phase 0, Task 0.2)."""
+
+from gateway.relay.descriptor import CONTRACT_VERSION, CapabilityDescriptor
+
+
+def _telegram_descriptor(**overrides) -> CapabilityDescriptor:
+    base = dict(
+        contract_version=CONTRACT_VERSION,
+        platform="telegram",
+        label="Telegram",
+        max_message_length=4096,
+        supports_draft_streaming=False,
+        supports_edit=True,
+        supports_threads=True,
+        markdown_dialect="markdown_v2",
+        len_unit="utf16",
+        emoji="\u2708\ufe0f",
+        platform_hint="You are on Telegram.",
+        pii_safe=False,
+    )
+    base.update(overrides)
+    return CapabilityDescriptor(**base)
+
+
+def test_descriptor_roundtrips_json():
+    d = _telegram_descriptor()
+    assert CapabilityDescriptor.from_json(d.to_json()) == d
+
+
+def test_descriptor_is_frozen():
+    d = _telegram_descriptor()
+    try:
+        d.max_message_length = 1  # type: ignore[misc]
+    except Exception as exc:  # FrozenInstanceError
+        assert "cannot assign" in str(exc) or "frozen" in str(exc).lower()
+    else:  # pragma: no cover
+        raise AssertionError("descriptor should be immutable (frozen)")
+
+
+def test_from_json_ignores_unknown_keys():
+    """A newer connector may send fields this gateway doesn't know — those are
+    dropped, not fatal (forward-compat during the experimental phase)."""
+    d = _telegram_descriptor()
+    raw = d.to_json()[:-1] + ', "future_field": "ignored"}'
+    restored = CapabilityDescriptor.from_json(raw)
+    assert restored == d
+
+
+def test_from_json_fills_optional_defaults():
+    """Optional fields (emoji/platform_hint/pii_safe) fall back to defaults."""
+    minimal = (
+        '{"contract_version": 1, "platform": "x", "label": "X", '
+        '"max_message_length": 2000, "supports_draft_streaming": false, '
+        '"supports_edit": false, "supports_threads": false, '
+        '"markdown_dialect": "plain", "len_unit": "chars"}'
+    )
+    d = CapabilityDescriptor.from_json(minimal)
+    assert d.pii_safe is False
+    assert d.platform_hint == ""
+    assert d.emoji == "\U0001f50c"
+
+
+def test_module_is_marked_experimental():
+    import gateway.relay.descriptor as m
+
+    assert "EXPERIMENTAL" in (m.__doc__ or "")