perf(start-client-core): zero-copy frame payload extraction (#7662)

extractFlattened() always allocated a new Uint8Array and copied `count`
bytes, even when the requested bytes were fully contained in the first
buffered chunk (the common case, since most frames arrive within a single
network read).

Add a fast path that returns a subarray view of the first chunk when the
bytes are contiguous, avoiding the allocation and the byte copy. The
multi-chunk path is unchanged. The returned view shares the chunk's
backing ArrayBuffer, which is safe because buffered chunks are never
mutated in place after being read.

This is on the hot path for decoding every streamed server-function
response and RawStream binary payload on the client. A micro-benchmark
shows ~3x faster extraction for 1KB frames and ~27x for 64KB frames (the
win scales with payload size).