Rendering glitches (flickering characters, incorrect fog/rain colors) on AMD GCN GPUs (RX 580) under Linux/RADV for the Shadow of the Colossus PC port. The core issue is the inability to engage the AMD 32-lane path due to requiredSubgroupSizeStages = 0.

Port version: v0.60.3, built from source on Linux (linux-port PR #39 merged)
Game: EU CUSA08809 + 1.01, base eboot (already carries the 1.01 bytes)
System: CachyOS (Arch), Mesa 26.2.4, RADV POLARIS10, AMD RX 580, Vulkan 1.3

**Symptoms** (intro area, stable 30 FPS otherwise):
- Wander, Agro and the colossi flicker green/pink/white every frame.
- Volumetric fog colours change on every launch with identical settings.
- Rain renders red.

**The port logs:**
```
GPU subgroups: size 64, min 64, max 64, required stages 0
```
On this GPU `IsSubgroupSize64Supported()` is false and `IsComputeSubgroupSize32Required()` is false (`minSubgroupSize == 64`), so `SubgroupSize()` returns 64 and `LowerWave64BallotPass` returns immediately. The AMD fix path described in the CHANGELOG (compute pinned to 32, wave64 lowering applied) never runs on GCN, because RADV reports `requiredSubgroupSizeStages = 0` for Polaris — there is no 32-wide wave to pin to.

**Settings verified inert here:** `SOTC_WAVE64_UNIFORM=0`, `SOTC_WAVE64_IDENTITY=0`, `SOTC_LDS_BARRIERS=1`. The subgroup line is identical with and without them, and the image does not change. `SharedMemoryBarrierPass` also exits because `needs_lds_barriers` is false at subgroup size 64 for RADV.

**Ruled out:** GPU/compute-queue synchronization. Running with `RADV_DEBUG=nocompute` (all compute serialized on the graphics queue) leaves the character flicker unchanged, so this is not a missing barrier between a dispatch and the draw that samples it...