feat(speech): add Vulkan whisper-tiny variants

[msluszniak]

Description

Registers the fp16 and int8 Vulkan exports of multilingual whisper-tiny as WHISPER.TINY.VULKAN_FP16 / WHISPER.TINY.VULKAN_INT8, adds vulkan to the speechToText entry in the download-libs.js feature map so the backend lib is fetched, and offers both in the speech demo on Android.

Encode runs on Vulkan (mel preprocessor falls back to XNNPACK); decode stays on XNNPACK, where it measured faster. On an Adreno 840 the encode is about 1.6x the XNNPACK time and decode is 4.08 ms, down from 11.96 ms.

The exports require the ExecuTorch 1.4.1 Vulkan runtime published in v0.10.0-libs-1.4.1. Older Vulkan libs lack the gelu tanh clamp, the reduce-shader barrier fix and the Adreno local-work-group fix, and produce NaNs or a device loss.

Introduces a breaking change?

• Yes
• No

Type of change

• Bug fix (change which fixes an issue)
• New feature (change which adds functionality)
• Documentation update (improves or adds clarity to existing documentation)
• Other (chores, tests, code style improvements etc.)

Tested on

• iOS
• Android

Testing instructions

1. Run apps/speech on an Android device.
2. Open Transcribe Audio File, pick Tiny Multilingual (Vulkan fp16), transcribe the sample file, and check the transcript carries no <|0.00|> timestamp tokens.
3. Repeat with Tiny Multilingual (Vulkan int8).
4. Repeat both on Live Transcription (Mic).

Screenshots

Related issues

Checklist

• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have updated the documentation accordingly
• My changes generate no new warnings

Additional notes

Verified on a Galaxy S26 Ultra against the exact libvulkan_executorch_backend.so published in v0.10.0-libs-1.4.1 (md5 3aba5c82..., matched between the installed APK and the release tarball).

The speechToText feature map lists xnnpack, coreml and now vulkan, but not mlx, even though the registry exposes MLX_BF16 / MLX_INT8 whisper variants. That looks like a pre-existing gap and is left alone here.

[msluszniak]

Folded into #1392, which already carries the registry variant wiring.