Add sim6502 - 6502 unit testing framework

[barryw]

Description

Adding sim6502 to the Cross / Tools section.

sim6502 is a unit testing framework for 6502 assembly code. It features:

• Custom DSL for writing tests against assembled 6502 code
• Works with KickAssembler symbol files
• Supports assertions, memory inspection, and CPU state verification
• Rich error messages with "Did you mean?" suggestions
• Test filtering, tagging, and skip options

Example test:

suites {
  suite("My Tests") {
    symbols("game.sym")
    load("game.prg", strip_header = true)
    
    test("score-increment", "Score should increase") {
      [score] = $00
      jsr([IncrementScore], stop_on_rts = true)
      assert(peekbyte([score]) == $01, "Score should be 1")
    }
  }
}

This fills a gap in the 6502 tooling ecosystem - there are many assemblers and emulators, but few dedicated testing frameworks.