Auto-detect WASM fallback for unsupported platforms (Termux/Android)

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## Problem
Sharp v0.35.2 fails to load on Termux (Android ARM64):
```
Error: Could not load the "sharp" module using the android-arm64 runtime
```

No prebuilt binaries exist for `android-arm64v8`.

## Workaround (verified working)
```bash
npm install @img/sharp-wasm32
```

## Proposed Solution
Auto-detect unsupported platforms and fall back to WASM:
```javascript
if (loadError && !process.env.SHARP_IGNORE_WASM_FALLBACK) {
try {
module.exports = require('@img/sharp-wasm32');
return;
} catch (_wasmError) {}
}
```

## Benefits
- Zero-config for unsupported platforms
- WASM is slower but functional
- Opt-out via `SHARP_IGNORE_WASM_FALLBACK=1`

## Related
- #3850, #4324, #4161

Start at the sharp module-loading path that emits the android-arm64 runtime error, and inspect how unsupported platforms are currently handled. Verify the proposed fallback with the @img/sharp-wasm32 package on Termux/Android, including the SHARP_IGNORE_WASM_FALLBACK opt-out; done means unsupported platforms load successfully without breaking supported ones.

Written by the indexing model from the issue text.