perf(eap-items): Encode RowBinary inserts in the processor

[phacops]

The RowBinary consumer kept Vec<EAPItemRow> alive in every batch from
the processor all the way to the writer. Each row carries ~80 attribute
buckets and their String allocations, so consumers ran with 2-3× the
in-flight memory of the JSONEachRow path and clickhouse-rs's Insert
builder double-buffered the encoded bytes during the write itself.

Each row is now serialized to RowBinary bytes inside
process_message_row_binary and the typed struct drops on the spot;
the pipeline carries only RowData { encoded_rows: Vec<u8>, num_rows }
the rest of the way. The writer is the same ClickhouseWriterStep the
JSON path uses, just with FORMAT RowBinary.

Vendored RowBinary serializer

clickhouse-rs's serializer is pub(crate) and Insert::write ties it
to an HTTP request, so there is no public surface to call it as
"serialize one row into a Vec<u8>". The new
strategies/clickhouse/rowbinary/ module is a ~280-line serde
Serializer covering exactly what EAPItemRow emits (LE primitives,
LEB128 string/seq length prefixes, raw-byte UUID with the half-reversal
ClickHouse's UUID type expects), with unit tests for each case.

Pipeline collapses to one shape

use_row_binary no longer takes a separate pipeline branch.
factory_v2.rs picks the processor function and the InsertFormat
once and the standard Reduce → ClickhouseWriterStep flow handles
both. TypedInsertBatch, EstimatedSize, the BytesInsertBatch<Vec<T>>
impl, make_rust_processor_row_binary, and
ClickhouseRowBinaryWriterStep are gone.

clickhouse-rs dependency dropped

The integration test (test_row_binary_clickhouse_insert) now exercises
the production path: it takes the processor's encoded bytes and POSTs
them with reqwest, then reads back via FORMAT JSON. With that, the
crate is removed from Cargo.toml and ~95 lines disappear from
Cargo.lock.

Agent transcript: https://claudescope.sentry.dev/share/ocWUXnBe7IomlfhNcOgtWgTf1Za2xq3Y5XMtqWqjSuo

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