restore selectAlternateInterface in camera worker

[turbocrime]

Closes #11.

Summary

• Restore selectAlternateInterface after claimInterface in the camera worker. The call was dropped in major refactor #4 (Oct 2025 refactor), which made Camera.setMode({ depth: ... }) crash the Chromium USB utility process on Linux with usb_device_handle_usbfs.cc Invalid argument (22). Linux usbfs needs an explicit SET_INTERFACE ioctl to reserve iso bandwidth before iso URB submission; macOS IOUSBLib was permissive enough to hide the regression.
• Alt setting is read from iface.alternate.alternateSetting (matches pre-refactor behavior; defensive against SKU/firmware variants vs. hard-coding 0).
• Extracted prepareCamDevice out of worker.ts so the claim → select-alt sequence is unit-testable in isolation (worker.ts has top-level self.addEventListener side effects that make direct import awkward). New tests pin the call ordering and descriptor-driven alt forwarding so this can't silently regress again.

Test plan

• pnpm --filter @webnect/driver build clean (tsc)
• pnpm --filter @webnect/driver test — 11/11 passing, 5 new under driver/test/prepare-cam-device.test.ts
• biome clean on changed files
• Manual: Linux Chromium + Kinect, Camera.setMode({ depth: ... }) streams depth without crashing the USB utility process; chromium-browser.log clean of Invalid argument (22)
• Manual: macOS Chromium smoke — depth + video still streaming (no regression from the helper extraction)

🤖 Generated with Claude Code

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🦋 Changeset detected

Latest commit: cdd9ac6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@webnect/driver	Patch
demo	Patch

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