Introduce v2 runtime and generator

[noxiouz]

Add a separate github.com/varlink/go/v2 module that can evolve independently from the v1 API while preserving the root module. The v2 design splits the implementation into explicit codec, wire, runtime, and generated API layers so framing, JSON handling, request semantics, and typed bindings can be optimized and reasoned about separately.

The internal codec now owns null-delimited framing, bounded frame reads, caller-owned buffer reuse, net.Buffers-based writes, and a JSON wire layer. It also supports both the default encoding/json backend and the experimental encoding/json/v2 backend behind GOEXPERIMENT=jsonv2. Frame and wire tests plus benchmarks cover small, medium, and large messages, fragmented reads, oversized frames, EOF behavior, and write-path allocation shape.

The v2 runtime adds explicit client and server APIs for unary calls, oneway requests, streamed replies, upgrades, and pipelined unary/oneway batches. Server registration is builder-based and immutable after Build, with separate handler types for unary, stream, and upgrade methods. The runtime includes built-in org.varlink.service support, service metadata, interface descriptions, typed remote errors, deadline-aware framed transport helpers, and certification-style coverage for the core varlink data shapes.

Add a lightweight v2 introspection package that extracts interface metadata and method modes from interface descriptions. Method mode is expressed through structured comments such as @mode stream, @mode oneway, and @mode upgrade, with unannotated methods defaulting to unary because plain varlink IDL does not encode transport mode. The certification IDL is annotated accordingly for streamed and oneway methods.

Add a v2 interface generator that reuses the existing IDL parser but emits github.com/varlink/go/v2/varlink bindings. Generated code now includes typed request/reply/error declarations, typed clients, typed batch helpers for unary and oneway calls, typed stream receivers, typed upgrade methods, mode-correct server handler adapters, interface descriptions, and remote-error dispatch. The legacy generator and parser also learn the official any type.

This commit intentionally keeps v1 code intact while creating a compileable v2 staging area. Remaining work is mostly around higher-level transport helpers, resolver integration, richer public schema APIs, external cross-language certification, and broader examples/CI coverage.

[cgwalters]

Since I'm trying to contribute to this repo (ref #44 ) I thought I'd spend some tokens reviewing this PR.

Assisted-by: OpenCode (Claude Opus 4.8 + Gemini 3.1 Pro)

AI-generated review. Comments prefixed with AI: are unedited AI output. Verified locally: the v2 module builds, go vet, go test, go test -race, and GOEXPERIMENT=jsonv2 go test ./internal/codec/... all pass.