Integer Overflow or Wraparound

Summary

Several Avro decoder paths read attacker-controlled 64-bit values from the wire format and either narrowed them to platform-sized `int` before bounds-checking, or summed them with overflow-prone signed-`int` arithmetic. On 32-bit targets (`GOARCH=386`, `arm`, `mips`, `wasm`, etc.), the truncation paths can silently bypass byte-slice limits, select the wrong union branch, or hit the OCF negative-`make` panic via wrap. Three sub-issues are not 32-bit-specific: cumulative-size arithmetic overflow in `arrayDecoder.Decode` / `mapDecoder.Decode` / `mapDecoderUnmarshaler.Decode` (wraps at `math.MaxInt64` on amd64 / arm64 and bypasses `MaxSliceAllocSize` / `MaxMapAllocSize`), `math.MinInt` negation in block-header handling, and `make([]byte, size)` with a negative size in OCF block reads -- all three panic or bypass caps on any platform, giving an attacker a denial-of-service primitive there.