is there an assembler for 6502, (and/or text editor) that is native to/runs on the 6502 itself?
Many.
e.g. The Apple II mini-assembler built into the Integer Basic ROM. Also there is a ROM one that was part of the AIM-65 setup. I'm sure there were many other ROM ones.
Ed. has mentioned in the land of Acorn - BBC Basic has a built-in assembler and as it's Basic, then it's also an editor. It's a powerful macro assembler too - as macros are Basic procedures.
Another acorn one was the assembler that came with the Acornsoft BCPL system. Choice of 2 editors there.
There was another I used on the Apple II - TED ][+ I think it was called.
And I started writing one for my own '816 BCPL system, but gave up for many reasons - mostly because I was happy using a cross assembler under Linux.
It wouldn't surprise me if there was one for the various DOS/65 projects either.
Not sure... I suspect if there were, then there would be a portable one out there already...
-Gordon