Unify whisper.cpp build release process across all AMD Hardware

[iswaryaalex]

This PR transforms whisper.cpp-rocm from a single-backend ROCm fork into a unified AMD build and release repository — covering ROCm, Vulkan, VitisAI NPU, and CPU across Windows and Linux.

What changes

New backends:

• Vulkan — cross-vendor GPU support (Windows + Linux)
• VitisAI NPU — RyzenAI 300 series NPU via FlexML runtime (Windows)
• CPU — baseline portable builds (Windows + Linux)

New source files:

• `src/vitisai/whisper-vitisai-encoder.h/.cpp` — FlexML runtime integration for NPU encoder
• `WHISPER_VITISAI` CMake option wired through `CMakeLists.txt`, `src/CMakeLists.txt`, and `src/whisper.cpp`

CI/CD overhaul:

• `build.yml` — unified release workflow across all backends and OS targets; fires on tag push and PRs targeting master
• `sync.yml` — daily upstream tracking; auto-merges and tags new whisper.cpp releases within 24h, no manual intervention needed
• `test-whisper.yml` — post-release verification; downloads built artifacts and runs whisper-cli against jfk.wav, checks transcription
  correctness

Release artifacts per version:
```
whisper-{version}-linux-rocm-{gfx_target}.tar.gz (gfx1151, gfx1150, gfx120X, gfx110X)
whisper-{version}-windows-rocm-{gfx_target}.zip
whisper-{version}-linux-vulkan-x86_64.tar.gz
whisper-{version}-windows-vulkan-x64.zip
whisper-{version}-windows-npu-x64.zip
whisper-{version}-linux-cpu-x86_64.tar.gz
whisper-{version}-windows-cpu-x64.zip
```

Other:

• `scripts/local-build.ps1` — reproduce any CI build locally on Windows
• `README.md` — rewritten for AMD release-focused audience with download tables and smoketest instructions
• All ROCm runtime libs bundled in artifacts — no separate ROCm installation needed

Key contributions on windows-rocm and Linux-rocm from @Geramy Key contributions on CPU perf optimization builds by @clemperorpenguin