Rev 1 — u16 revision || Vec<Value> (length-prefixed). Byte-identical to the legacy
on-disk encoding.Rev 2 — optimised envelope (u16 revision || u32_le payload_length), inner Vec written
via the indexed-seq prologue past OFFSET_TABLE_MIN_LEN = 8. Walker descent stays
zero-allocation through the Wire-repr fast path (skip + borrow). Walker exposes
element_bytes(i) for O(1) random access on the indexed path; sub-threshold arrays fall back
to a linear walk of the legacy body.