NES: FDS Accuracy Improvements

[TakuikaNinja]

This is a redo of #85 which avoids changing the current audio implementation.

Working

• $4023.D0 only affects writable disk registers, forces certain values for $4025 and $4026
• $4023.D1 only directly affects $4080 (volume envelope), $4085 (mod counter), and $4088 (mod table write), apply instant volume mute for now
• External connector: Mask $4026 writes to 7 bits, and force bit 7 = 1 for $4033 reads (i.e. always return good battery/power status)
• $4030 reads do not clear byte transfer flag/disk IRQs
• Move byte transfer flag to $4030.D7 (eventually needed by Tonkachi Editor)
• $4090-$4097 reads (debug audio registers), handled by manually calculating the necessary values this time
• Add remaining registers to register view
• Fix off-by-one in envelope timer init

Not addressing

• Register peeking
• Actual audio reset state when $4023.D1 = 0 (research ongoing) + rename _soundRegEnabled to reflect its purpose
• $4095 upper nybble
• DRAM refresh watchdog ($4030.D1)
• CRC handling ($4025.D4 & D6)
• End of head behaviour ($4030.D6)
• Byte transfer flag behaviour ($4030.D7)
• Wavetable volume/modulation accuracy improvements
• Non-linear DAC + volume PWM + filtering (probably not worth the performance hit for a minuscule audio improvement)

Test programs

• FDS-Mirroring-Test (Passes all tests on latest version)
• FDS-4023-Test (Readable registers stay readable)
• FDS-Audio-Registers (Audio plays properly, albeit with inaccurate audio reset state)

[TakuikaNinja]

Here's how the register view currently looks: