Closes #83.

View types had no JSON serialization path. Users holding a decoded FooView<'_> had to call .to_owned_message() and allocate a full owned message just to serialize to JSON — the whole point of views is to avoid that allocation.

Problem

generate_json already wired up #[derive(Serialize, Deserialize)] on owned message structs and enums. Views were left out because they require a manual impl rather than a derive: field access goes through getter methods, map fields need a custom iterator wrapper, and the proto3 JSON rules (omit defaults, quote int64/uint64, base64-encode bytes, encode NaN/Inf as string tokens, NullValue as JSON null) need explicit handling at codegen time. There was no generate_view_serialize function in buffa-codegen/src/view.rs.

Fix

Added generate_view_serialize in buffa-codegen/src/view.rs. When both generate_views and generate_json are enabled for a file, each generated *View<'a> struct gets a manual impl<'__a> serde::Serialize for FooView<'__a> that opens a serde map, iterates fields, and applies the same proto3 JSON rules as the owned-side impl: proto3 defaults are skipped, bytes go through json_helpers::bytes_serialize, int64/uint64 through json_helpers::int64_serialize, enum values emit their proto name, and NullValue oneof variants emit JSON null via serialize_entry(key, &()).

Map fields use a local _WM newtype that iterates the MapView and calls the appropriate key/value helpers inside a SerializeMap; the pattern mirrors the existing owned-message approach. Scalar fields needing a helper emit a local _W newtype inside a block scope — this scope is required when a message has more than one helper-typed required field (proto2), because block scoping prevents the duplicate-definition error that would otherwise result from two struct _W declarations at function scope.

OwnedView<V> gains a blanket impl<V: serde::Serialize> serde::Serialize for OwnedView<V> (cfg-gated on feature = "json") in buffa/src/view.rs, so serde_json::to_string(&owned_view) works without an explicit &*owned_view deref.

Known limitations (documented in CHANGELOG and in the generated impl's doc comment): messages whose view types nest a WKT view (TimestampView, DurationView, etc.) fail to compile because WKT view types do not yet implement Serialize. The workaround is to call .to_owned_message() and serialize the owned form; fixing the WKT side is a planned follow-up. Extension fields are also not included in view JSON output.

Tests

Four codegen integration tests cover the code-generation path: one verifies the impl serde::Serialize block is present when generate_json is enabled, one verifies it is absent when disabled, one checks that json_helpers calls appear for bytes and int64 fields, and one is a regression test for the proto2 required-field _W-collision fix (verifies the emitted token stream parses without E0428).

Nine round-trip tests in buffa-test/src/tests/json.rs exercise the generated impls end-to-end against view_json.proto (a dedicated proto file with no WKT message imports): scalar fields including int64 quoting and bytes base64, double special values (NaN/Inf/-Inf), proto3 default omission, enum name encoding, all oneof variants including NullValue, map fields with int32 keys and int64 values, nested messages with repeated fields, and the OwnedView<V> blanket impl. Each test decodes a wire-encoded message into a view and compares serde_json::to_string(&view) against serde_json::to_string(&owned) to confirm parity.