child_process: frame advanced IPC messages natively

[anonrig]

Summary

Move the advanced child_process IPC read-path framing into C++,
continuing the child_process JS→C++ migration.

The advanced codec previously reframed every read in JavaScript
(parseChannelMessages in lib/internal/child_process/serialization.js):
scanning the 4-byte big-endian length prefix, accumulating partial frames in
an array, and Buffer.concat-ing messages that span reads, then crossing
into the ipc_serdes binding once per message to deserialize.

This adds a native per-channel IPCChannelFramer (a BaseObject on the
ipc_serdes binding) that owns the cross-read accumulation buffer in C++ and
returns an array of complete, deserialized messages per read, so JavaScript
no longer reframes the byte stream. Messages contained entirely within a read
are deserialized in place and let host objects borrow the read buffer (zero
copy); only frames that span reads are copied (matching the previous
Buffer.concat behavior).

Invariants preserved

• Wire format is byte-identical (4-byte BE length prefix + V8 payload with
  the custom host-object tags).
• Handle passing is untouched: the per-read pendingHandle handoff and
  NODE_HANDLE delivery stay in setupChannel.
• Public events and their timing are unchanged.

json stays in JavaScript (deliberate)

I implemented and benchmarked native json framing as well, but it
regressed the json read path by up to ~40% on large messages: its
StringDecoder + split('\n') pipeline is already copy-free (V8 rope
concatenation and O(1) substrings), whereas reframing in C++ adds a buffer
copy plus a per-message String::NewFromUtf8. Only advanced is framed
natively; the json codec is left unchanged.

Benchmarks

benchmark/child_process/child-process-ipc-roundtrip (advanced, msgs/sec;
interleaved A/B vs. the pre-change baseline, 8 reps × dur=5s):

payload	before	after	delta
1 KiB	559,040	626,255	+12.0% (t=5.3)
4 KiB	311,284	330,153	+6.1% (t=2.3)
16 KiB	104,077	125,181	+20.3%
64 KiB	31,921	35,739	+12.0%

json round-trip and read throughput are unchanged (a json control row in the
same harness lands within the noise floor).

Tests

• New cctests in test/cctest/test_node_ipc_serdes.cc: single-message
  round-trip, several messages delivered in one read, and a frame split across
  two reads (including a split length header).
• test/parallel/test-child-process-*, test/parallel/test-cluster-*, and
  test/sequential/*child* pass locally.

[anonrig]

Benchmark CI: https://ci.nodejs.org/view/Node.js%20benchmark/job/benchmark-node-micro-benchmarks/1876/

[anonrig]

Config	old (ops/s)	new (ops/s)	Δ	sig
advanced len=64	467,991	488,482	+4.4%	***
advanced len=256	399,774	411,011	+2.8%	***
advanced len=1024	321,322	336,254	+4.7%	***
advanced len=4096	210,270	225,204	+7.1%	***
advanced len=16384	91,388	104,764	+14.6%	***
advanced len=65536	21,868	21,574	-1.3%	***
json len=64	692,902	689,566	-0.5%	—
json len=256	519,877	513,564	-1.2%	***
json len=1024	334,806	332,229	-0.8%	**
json len=4096	161,677	160,458	-0.7%	**
json len=16384	57,085	56,531	-1.0%	***
json len=65536	13,297	13,161	-1.0%	—