fix: rewrite decode_struct so struct decoding actually works

[medvednikov]

Summary

Fixes #12. Struct decoding never worked: the previous decode_struct

• never read the map header byte,
• never read the field-name keys the encoder writes (it tried to read values directly),
• called d.decode(data[d.pos..], mut field_val) — re-slicing the buffer without resetting d.pos, so next() read from the wrong offset,
• and assigned via val[field.name] = field_val, which doesn't compile against current V (`type 'T' does not support indexing`).

The user-visible symptom in #12 was `invalid integer descriptor byte 117` — 0x75 = 'u', the first byte of the key \"used_ids\" being misinterpreted as an integer descriptor.

Rewrite to mirror the encoder's wire layout:

1. Read the map header (already loaded into d.bd by the prior next()) → map_len.
2. For each entry: next() → decode the key as a msgpack string → next() → look up the matching field by name (honoring @[codec: 'alias'] attrs the encoder writes) → decode the value into it via comptime field assignment.

Extracted the type-dispatch out of decode[T] into a private decode_value helper so containers can decode children without clobbering d.buffer / d.pos.

Also uncomments the previously-disabled struct round-trip assertion in decode_test.v and adds round-trips that don't depend on the encoder's chosen integer width.

Test plan

• v test . — all 3 test files pass (encode, decode, decode_to_json), including the previously-commented struct round-trip
• Issue Decoding does not work at all. #12 repro now succeeds: msgpack.decode[Map](msgpack.encode(Map{used_ids: 1})) returns Map{used_ids: 1} instead of erroring with byte 117
• Round-trip a struct with mixed-type fields: Struct{'John', 30} and Struct{'', 0}
• Independent of fix: use minimal integer encoding and decode fixint bytes #13 (this PR branches from master)