APU Icarus testbench (issue #398)

Bring-up + regression for the real APU netlist of the DMG-CPU (HDL/soc/apu.v), mirroring the PPU (issue #390) and small-domain (issue #396) suites.

The suite drives the APU in its SoC context: real ClkGen + MMIO (mmio_weakbus) + Arbiter + Ser + a behavioral WaveRAM (wave_ram_model.v: the repo macro HDL/soc/waveram.v is an empty stub), with a CPU bus master using the SoC bus conventions of ../soc (data on d while clk2=1, capture as the FFxx write window closes).

Tests (all PASS in run_all.sh)

Test	Purpose
`tb_apu_regs`	Register-file map of $FF10-$FF3F (read/write masks, NR52 power semantics, wave-RAM window decode)
`tb_apu_ch1` + `check_ch1.py`	CH1 square: period = (2048-X)*32 osc (131072/(2048-X) DMG formula), duty 12.5/25/50/75%, volume plateau, NR52 status
`tb_apu_ch2` + `check_ch2.py`	CH2 square: same set of checks
`tb_apu_ch3` + `check_ch3.py`	CH3 wave: sample period = (2048-X)*2 osc, amplitude = NR32-scaled wave sample (100/50/25%/mute), wave-a stepping
`tb_apu_ch4` + `check_ch4.py`	CH4 noise: LFSR output toggles at vol F; NR43 divider rate dependency

Research probes kept for the record: tb_apu_probe/tb_apu_readwin/ tb_apu_readsel/tb_apu_regmask (register decode/read timing), tb_apu_fs (frame-sequencer rate measurements with the synthetic LFO).

Files

apu_merged.v - the APU netlist with the a/d bus aliases merged (regenerate: python3 ../soc/merge_bus_aliases.py ../../apu.v apu_merged.v --buses a d)
apu_wc.v - write-clean variant for Icarus (regenerate: python3 make_wc.py): gates the w548 divider-state d-drivers (g869/g937/g939-941) off during CPU writes; without this the level-sensitive divider preset latches capture x from the write-bus contention (see STATUS.md)
apu_env.v - reusable environment (real DUTs + CPU model + synthetic LFO override for frame-sequencer measurements)
wave_ram_model.v - behavioral 16-byte WaveRAM
apu_trace.py - netlist decode/cone tracer (research tooling)
check_ch*.py - waveform metric analysers (period/duty/amplitude)
waves.md - waveform documentation (per-test images)
STATUS.md - research log & open questions

Compile line: iverilog -D ICARUS -o <t>.run ../../dmglib.v ../../clkgen.v ../soc/mmio_weakbus.v ../soc/arb_merged.v ../soc/ser_sharedq.v apu_wc.v wave_ram_model.v apu_env.v <t>.v (WSL-native Icarus; see ../soc notes).