TitleScreen crash: isb $f3d3,x at $1:f56c writes to read port f3d4

**Crash details:**
- Location: $1:f56c contains `isb $f3d3, x` (illegal opcode)
- Error: Write to read port f3d4 (TIA read register)
- Bank: Bank 1
- Instruction: ISB (Increment and Subtract with Borrow) - illegal opcode

**Context:**
- Occurs after fix for issue #1219 (titledrawscreen symbol resolution)
- Stack: s f3 ( fd c1 c4 f1 2c 54 11 8a ed fa a3 fa ) a 6a x 01 y fb ps -----i-c
- Memory at $00f2: 6b f5 fd c1 …

**Investigation needed:**
- Why is illegal opcode ISB being generated?
- What code is at $1:f56c?
- Why is it trying to write to TIA read register f3d4?
- Is this related to the titlescreen_colors.s rorg change?