Build coding paths lazily and read arrays in place

[letko-dmitry]

Builds on #63: only the last 2 commits are new, and either can be dropped.

• Coding paths are built only when an error needs them, not for every nested value (−6% encoding, −4% decoding).
• Arrays are read in place instead of being copied into [Any?] up front (−21% decoding). Elements that are optionals themselves unwrap as before, and a test covers it.

Behavior doesn't change. Together with #61–#63, encoding is 27% faster than on main and decoding 35%.

Measured in retired instructions with alternating A/B runs, release builds on an M4; noise is within ±0.3%.