I’ve seen a lot of software and IDEs over time, but which one produces/uses .DSD files?

The new C64 source is a 400KiB(!) sized .DSD file which contains a weird mix of ASCII and binary. The beginning of the file seems to indicate that it is a kind of container file, but it definitely is not a “Direct Stream Digital” audio file ;-)

I can see 6510 assembler instructions in the hexdump of the file, but without any spaces or line breaks (just out of the middle of the file where I’m currently looking at):

0000c8a0  44 41 58 58 33 2b 33 2c  58 3a 53 54 41 58 58 31  |DAXX3+3,X:STAXX1|
0000c8b0  32 2b 31 3a 4c 44 41 58  58 33 2b 32 2c 58 3a 53  |2+1:LDAXX3+2,X:S|
0000c8c0  54 41 58 58 31 32 0d 22  6a 33 4c 44 41 58 58 33  |TAXX12."j3LDAXX3|
0000c8d0  2b 31 2c 58 3a 53 54 41  58 58 31 35 2b 35 3a 4a  |+1,X:STAXX15+5:J|
0000c8e0  53 52 4c 4c 31 34 37 3a  42 43 53 4c 4c 37 39 2d  |SRLL147:BCSLL79-|
0000c8f0  33 3a 4a 4d 50 4c 4c 38  30 0d 22 74 10 2e 4c 4c  |3:JMPLL80."t..LL|

There clearly are 6502/6510 instructions: STA, LDA, JSR, BCS and JMP, but no spaces between the instruction and the parameter, and it seems to use either a colon (:) as a command separator or an occasional 0x0d followed by three other characters (probably for larger sections?)

Any ideas how to unpack this into a readable source?