Add AMD ROCm GPU support

Introduces a third hardware mode alongside CUDA and CPU: ROCm (AMD GPU).
AMD GPUs present as CUDA devices under PyTorch ROCm, so the existing GPU
path is reused with minimal changes — the main additions are wheel management,
device detection, and suppressing flash_attn (unsupported on ROCm).

- server/vibevoice_server.py: extend _resolve_device() to recognise 'rocm'
  (auto-detected via torch.version.hip); add _torch_device() helper that maps
  'rocm' → 'cuda' for all PyTorch API calls; apply GPU optimisations for both
  cuda and rocm in _init_model(); always use sdpa on ROCm; propagate
  _torch_device() to _load_voice_presets() map_location.
- server/start.sh: add --rocm flag; sync .venv-rocm with uv sync --no-sources
  then replace torch with the ROCm 6.2 wheel via uv pip install; set
  VIBEPOD_DEVICE=rocm for uvicorn.
- server/pyproject.toml: register pytorch-rocm62 index (explicit); add
  .venv-rocm to ruff excludes.
- package.json: add dev:rocm and dev:server:rocm scripts.
- README.md: document ROCm mode, prerequisites (RX 6000+, ROCm 6.2+, Linux),
  and new commands; expand CUDA vs CPU section to CUDA vs CPU vs ROCm.

https://claude.ai/code/session_0168pSswiaoEf6LEx6UQWfBu

server/start.sh

@@ -6,15 +6,17 @@
 # Usage:
 #   ./start.sh          — CUDA mode (default, uses PyTorch CUDA 12.4 wheel, venv: .venv)
 #   ./start.sh --cpu    — CPU-only mode (uses PyPI CPU torch wheel, venv: .venv-cpu)
+#   ./start.sh --rocm   — ROCm mode (AMD GPU, uses PyTorch ROCm 6.2 wheel, venv: .venv-rocm)
 #
 # Optional CUDA acceleration:
 #   VIBEPOD_ENABLE_FLASH_ATTN=1 ./start.sh
 #   Installs a pre-built flash-attn wheel when the CUDA venv uses Python 3.12,
 #   torch 2.6.0, and CUDA 12.4 on Windows. Other platforms fall back to SDPA.
 #
-# The two modes maintain completely separate virtual environments so their torch
+# The three modes maintain completely separate virtual environments so their torch
 # installations never conflict. UV_PROJECT_ENVIRONMENT tells uv which venv to use;
-# --no-sources skips [tool.uv.sources] so the CPU run pulls the default PyPI torch wheel.
+# --no-sources skips [tool.uv.sources] so the CPU/ROCm run pulls the default PyPI torch
+# wheel first, then torch is replaced with the appropriate wheel for that mode.
 
 set -euo pipefail
 
@@ -25,12 +27,14 @@ cd "$SCRIPT_DIR"
 # Parse flags
 # ---------------------------------------------------------------------------
 CPU_MODE=false
+ROCM_MODE=false
 PASSTHROUGH_ARGS=()
 
 for arg in "$@"; do
     case "$arg" in
-        --cpu) CPU_MODE=true ;;
-        *)     PASSTHROUGH_ARGS+=("$arg") ;;
+        --cpu)  CPU_MODE=true ;;
+        --rocm) ROCM_MODE=true ;;
+        *)      PASSTHROUGH_ARGS+=("$arg") ;;
     esac
 done
 
@@ -38,6 +42,8 @@ echo "================================================"
 echo "  VibePod TTS Server"
 if $CPU_MODE; then
     echo "  Mode : CPU-only"
+elif $ROCM_MODE; then
+    echo "  Mode : ROCm (AMD GPU)"
 else
     echo "  Mode : CUDA (default)"
 fi
@@ -89,6 +95,21 @@ if $CPU_MODE; then
         cp "$LOCK_BACKUP" uv.lock
         rm -f "$LOCK_BACKUP"
     fi
+elif $ROCM_MODE; then
+    echo "--> Syncing ROCm Python environment (.venv-rocm)..."
+    export UV_PROJECT_ENVIRONMENT=".venv-rocm"
+    LOCK_BACKUP=""
+    if [[ -f uv.lock ]]; then
+        LOCK_BACKUP="$(mktemp)"
+        cp uv.lock "$LOCK_BACKUP"
+    fi
+    uv sync --no-sources
+    echo "--> Installing PyTorch ROCm 6.2 wheel..."
+    uv pip install torch --index-url https://download.pytorch.org/whl/rocm6.2
+    if [[ -n "$LOCK_BACKUP" ]]; then
+        cp "$LOCK_BACKUP" uv.lock
+        rm -f "$LOCK_BACKUP"
+    fi
 else
     echo "--> Syncing CUDA Python environment (.venv)..."
     uv sync
@@ -166,6 +187,10 @@ if $CPU_MODE; then
     #   VIBEPOD_COMPILE=1       torch.compile hot paths (ineffective for autoregressive
     #                           models on CPU — not recommended, kept for experimentation)
     UV_RUN_ARGS=(--no-sync --no-sources)
+elif $ROCM_MODE; then
+    export VIBEPOD_DEVICE="rocm"
+    export UV_PROJECT_ENVIRONMENT=".venv-rocm"
+    UV_RUN_ARGS=(--no-sync --no-sources)
 else
     export VIBEPOD_DEVICE="cuda"
     UV_RUN_ARGS=()