Update APU and MMIO specifications

[rafael84]

Motivation

• Bring the documentation in-line with the current emulator/APU implementation by clarifying the APU register layout, waveform behavior, and data formats.
• Replace outdated/ambiguous MMIO ranges and TODOs so integrators and toolchains can rely on a stable MMIO map and APU spec.

Description

• Updated specs/tiny16-apu.txt to describe the unified 4-channel APU design, added the complete waveform list and precise waveform behavior, expanded the note table (C2-B6), and added SFX and music data formats.
• Updated specs/tiny16-vm.txt to reflect the MMIO region size as 4 KB (0xBF00-0xBFFF) and to reference the APU register block at 0xBF40-0xBF9F while reserving 0xBFF0-0xBFFF for future system registers.
• Added register-level details including per-channel register stride, music and SFX registers, example usage, and removed obsolete TODOs and legacy channel descriptions.

Testing

• No automated tests were run because this is a documentation-only change (not applicable).

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Codex Task