docs: extend IR to the full cross-format capability surface

[OmarAlJarrah]

Summary

The IR draft had gaps that would have forced schema changes the moment later frontends landed — violating the core rule that only frontends are staged over time while the IR capability surface is complete from day one. This revision closes them, based on an audit of the model against the official specs for Smithy 2.0, TypeSpec (language + http/streams/SSE/events/versioning/rest/xml libraries and the current visibility system), Protobuf (proto2/proto3/editions, gRPC + transcoding), GraphQL (October 2021 + working draft), AsyncAPI 3.0, and OpenAPI 3.1/3.2, plus a representability sweep over Thrift, Avro, JSON-RPC/OpenRPC, WSDL, and Cap'n Proto. It also adds Erlang/OTP as a supported frontend target, scoped to the message-protocol surface (gen_server/gen_statem/gen_event call/cast/info, Dialyzer type language).

Headline changes to docs/ir-design.md (normative):

• One-way operations — Operation.OneWay distinguishes fire-and-forget (OTP cast, AsyncAPI send-without-reply, Thrift oneway, JSON-RPC notifications) from "response with no body".
• Messages get identity — MessageID + Document.Messages registry (they were embedded by value, contradicting the no-embedded-named-nodes rule, and MessageRef was referenced but never defined); AsyncAPI request-reply gains Reply with static channel + dynamic reply address + reply message set; message headers become a typed schema.
• Erlang/OTP support — new OTPBinding (behaviour, call/cast/info, target process as a channel, request tag); positional discrimination (Discriminator.Index) for tagged-tuple unions; Model.Positional for records; symbol values for atoms.
• Services gain IDs, inheritance (Extends), per-service versions/renames, and protocol declarations; servers gain names, server-scoped auth (AsyncAPI's primary placement), and protocol versions.
• OpBindings.HTTP becomes a slice — gRPC transcoding additional_bindings needs N HTTP mappings per operation; bindings also gain transcoding path patterns, response_body projection, compression/checksum obligations, and shared routes.
• Property gains Presence (protobuf implicit/explicit/required — not nullability), ClientOptional/DefaultAdded (Smithy evolution model), EventPayload; WireID becomes *int (0 is a legal ordinal in Cap'n Proto/FlatBuffers/Avro).
• Visibility.None (@invisible), operation-level visibility overrides, type-level XML/wire-name/media-type hints, richer Example (input↔output/error scenarios, header+payload pairs), Availability on enum members/variants/parameters with add/remove cycles and optionality flips.
• AuthKind covers the SASL/X509/userPassword families; Extensions/Provenance now exist on every node that can carry source metadata (responses, contents, examples, flows, bindings, auth schemes).
• Documented conventions for degraded constructs (§4.8): TypeSpec never/string templates, Erlang bit-sized binaries and funs, Smithy waiters and endpoint rules, GraphQL directive ordering and @oneOf lowering.

docs/ir-spec-matrix.md gains the Erlang/OTP column, six new capability rows (one-way ops, positional encoding, symbol literals, unsolicited messages, multi-format payload schemas, extension ranges), and ~25 cell corrections (e.g. Smithy optionality-vs-nullability was overstated, protobuf reserved is not versioning, GraphQL has no server construct). architecture.md and CLAUDE.md add OTP to the format lists.

Test plan

• Docs-only change; no code exists yet (design stage).
• Verified internal consistency: every struct/field referenced by the lowering summaries (§14) and matrix exists in the model; no references to removed shapes (MessageRef, ReplyTo, FilenameParam, bool XML node flags) remain.
• When Milestone 1 lands, the capability-conformance corpus (one minimal spec per matrix row per format) is the executable check for these claims.