Adds `write_message::encode_into_bytes_mut` (and the timestamped
variant) which writes the encoded body into a caller-supplied
`BytesMut`. The buffer is cleared and resized in place each call;
once it has grown to the largest body the session will produce, it
allocates nothing further.
A session that holds a single `BytesMut` and reuses it across writes:
- Int32 / Float32 / Float64: 2 → 1 allocs/op
(only the `encode_scalar_value` scratch `Vec<u8>` remains)
- Boolean: 1 → 0 allocs/op
(no per-value scratch — the literal payload is a stack `[u8; 4]`)
Bench delta in `design/M6-bench-baseline.md` § F52.3. The
`encode_scalar_value` Vec is the remaining 1 alloc/op for fixed-width
scalars; eliminating it would require inlining the LE-bytes write
into the body slice (left for a follow-up since the F52 spec only
asks for 2 → 1).
Resolves F52 (all three optimisations landed: 4e76b44 F52.1,
a0fa5be F52.2, this commit F52.3). Existing `encode` / `encode_to_bytes_mut`
public surface unchanged.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
Joseph Doherty
ceeaeefa71