perf(parsers): transcode protobuf wire bytes straight to proto3 JSON

Protobuf.Schema.parse and parseStream now decode protobuf bytes directly to a
proto3-JSON string against the descriptor (pb_transcode), skipping the
intermediate prost_reflect DynamicMessage tree + serde serialization. One pass,
length-delimited values referenced in place.

This is both faster and lighter:
  parse (50k-element catalog):  ~6.0s -> ~4.6s   (gap to node 5.3x -> 4.0x)
  whole-parse peak RSS:         ~100 MB -> ~63 MB

Output is identical to the previous prost-reflect serialization — a differential
unit test (transcode_matches_prost_reflect) pins byte-for-value parity across
every scalar type, enums, nested messages, repeated (packed + message), and maps.
Suite 80/80; protobuf round-trip/advanced/stream conformance unchanged.

The remaining gap to node is the JSON-string bridge into V8 (JSON.parse, kept
deliberately — direct Value marshaling is slower) plus per-message bucketing.

Author: Thanga-Ganapathy

CHANGELOG.md

@@ -6,6 +6,15 @@ pre-`0.1.0` and the public API is unstable.
 
 ## [Unreleased]
 
+### Changed
+
+- **`runtime:parsers` — faster, lighter Protobuf decode.** `Protobuf.Schema.parse`
+  (and `parseStream`) now transcode protobuf wire bytes straight to proto3 JSON
+  against the descriptor, without building an intermediate `prost_reflect`
+  `DynamicMessage` tree. On a 50k-element catalog: parse ~6.0s → ~4.6s and
+  whole-parse peak RSS ~100 MB → ~63 MB. Output is unchanged (a differential test
+  pins byte-for-value parity with the previous `prost-reflect` serialization).
+
 ### Fixed
 
 - **`runtime:parsers` — TOML datetimes** now parse to RFC3339 strings instead of