[Kinflou]

Stacked on #31. Stage 1 of runtime#29.

• TypeSeed decodes bytes by a Type known only at run time, making exactly the calls a serde derive makes, through each format's decode_seed (Json and MsgPack decode through a seed, as Packed does #31). Before this, the reference could decode only three hand-written reader types, and every other shape was NotImplemented.
• Type now covers every shape generated Rust has: all scalars up to 128 bits, unit, option, T[], and T[N] (including N > 32, which needs serde_big_array), plus struct and enum. Value gains u128, i128 and unit. Unions are left out, since the Rust generator refuses them.
• tests/seed_vs_derive.rs holds TypeSeed to real derives: the old readers plus the generation goldens' shapes (Scalars, Bag, Sample, error structs, unit). Each format is tried on valid bytes, every single-byte mutation, every truncation, a trailing byte, and hand-written JSON and MessagePack maps. Three deliberately planted seed bugs each fail it.
• Names go through an interner instead of Box::leak on every encode, so a long-running comline-reference process doesn't grow.
• answer() asks the reference one question, for the binary in the next step.
• Vectors: 14 new cases per format for the new shapes. No existing case changed (diffed by id).

One finding, left as it is here: msgpack@1 accepts trailing bytes after a value, because rmp_serde never checks the end of input. packed@1 and json@1 refuse them. That's a strictness gap in the reference itself.