feat: read-only getDevices() to list paired glasses

[gdelataillade]

Summary

Adds a read-only MetaWearablesDat.getDevices() returning List<WearableDevice> so apps can enumerate the paired glasses, tell two pairs of the same model apart (by name/id), and see which pair is actually streaming.

This is step 1 of 2 — read-only listing. Step 2 (pinning a specific pair via the SDK's SpecificDeviceSelector, keyed on id) will follow.

What's included

• Dart: WearableDevice + WearableDeviceType / WearableLinkState / WearableCompatibility enums (tolerant fromMap, non-empty id required); facade, platform interface, method-channel decode.
• iOS: getDevices over Wearables.shared.devices + deviceForIdentifier (complete fallback on nil); canonical codes with @unknown default on the non-frozen DeviceType/Compatibility.
• Android: getDevices over Wearables.devices / devicesMetadata with a NOT_INITIALIZED guard; sessionDeviceId captured before createSession and cleared on teardown.
• Example app: PairedDevicesSheet + Devices FAB (visible while streaming); provider refreshDevices() with coalescing, name/id sort, and refresh on active-device + session-state transitions.
• Tests: the repo's first Dart unit tests (model fromMap, enum fallbacks, channel decode incl. null→[] and PlatformException, facade delegation).
• Docs: CHANGELOG 0.6.0, README, AGENTS.

Key design notes

• isActive (the auto-selector's current pick — what a new stream would bind to) is distinct from isStreamingDevice (the pair the live stream is using). They can differ with 2+ connected pairs, and the device session outlives the camera stream — so isStreamingDevice reads the actual Stream.state == streaming rather than a possibly-stale non-null reference.
• id (DeviceIdentifier) is the canonical identifier and the future pinning key; no deviceUUID is exposed (it's iOS-only and not the selection key).
• Canonical lowerCamelCase codes are emitted identically on iOS and Android.

Verification

• dart analyze clean; 16 Dart unit tests pass.
• flutter build apk ✓ and flutter build ios --no-codesign ✓.

Release note

Core is bumped to 0.6.0; the flutter_meta_wearables_dat_mock_device add-on is still at 0.5.3 — align it at release time. Step 2 (device pinning) can fold into the same unreleased 0.6.0.

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