Streaming RawJson deserialize + hot-path cleanups (1.15-1.60x round-trip)

[jkschneider]

Summary

Two compounding optimizations on the hot path of io.moderne.jsonrpc, the IPC layer for OpenRewrite multi-language work. Round-trip end-to-end measured on a paired-pipe dispatch loop (5000 iterations):

Payload	Baseline	This PR	Speedup
tiny	48.9 μs	42.4 μs	1.15×
medium	43.1 μs	35.7 μs	1.21×
large (3.5KB)	168 μs	105 μs	1.60×

Caveat: artificial bench. Real-workload (JFR on a real OpenRewrite recipe) deferred — see Test plan.

Phase 2 — RawJson wrapper + streaming deserialize

• New io.moderne.jsonrpc.RawJson: library-owned wrapper holding either a Jackson TokenBuffer (inbound, lazy materialization) or a POJO (outbound). Keeps Jackson out of the public ABI.
• JsonRpcRequest.params and JsonRpcSuccess.result change from Object to RawJson. JsonRpcRequest.newRequest(method, POJO) factory keeps working.
• JsonMessageFormatter.deserialize: full streaming via JsonParser + TokenBuffer, no intermediate JsonNode tree. convertValue unwraps RawJson for either POJO or TokenBuffer.

The streaming path moves cost from RECEIVE → DISPATCH (deferred conversion). On large payloads RECEIVE p50 dropped 64.9 → 46.6 μs (1.39×) while DISPATCH p50 rose 18.8 → 22.7 μs — net win is the streaming saving.

Phase 3 — Hot-path cleanups

• JsonRpcMethod — paramType resolved once in the constructor instead of per dispatch via getClass().getGenericSuperclass().
• JsonRpcIdDeserializer — direct JsonToken inspection, no JsonNode allocation per inbound id.
• internal/SnowflakeId — lock-free generation via single packed AtomicLong (timestamp << SEQUENCE_BITS | sequence), preserving millisecond monotonicity. New concurrency test in SnowflakeIdTest.
• HeaderDelimitedMessageHandler / NewLineDelimitedMessageHandler — wrap input in BufferedInputStream when the caller hasn't already.
• MeteredMessageHandler — pre-build the four no-error Timers in the constructor; cache error-tagged Timers in a bounded ConcurrentHashMap (cap 64) to avoid the cardinality trap of unbounded per-method tags.

ABI note

JsonRpcRequest.params and JsonRpcSuccess.result are now typed RawJson rather than Object. Consumers that previously cast to Map will get a deterministic compile-time error rather than silent misbehavior. Major-version bump expected.

Test plan

• ./gradlew test passes
• SnowflakeIdTest confirms no duplicate ids under concurrent generation
• Existing JsonRpcTest round-trip suite passes against the streaming deserializer
• JFR profile on a real OpenRewrite multi-language recipe (deferred to a follow-up — gate criterion for any subsequent MessagePack work)