Emit device ids in lower-case on every platform (breaking, next major)

[postmaxin]

Follow-up to #269 (merged in 2.1.1): make the emitted device-id case consistent too — the fully-universal, breaking version discussed there for the next major.

What changes

Device ids are now canonicalised to lower-case throughout the Dart layer and emitted lower-case on every platform (scan results + connection / value / pairing / connection-parameter callbacks and streams). Previously each platform reported its native case — Android upper-cased MACs, Windows/WinRT lower-cased them — so a caller holding the id in the "wrong" case could split state or (before #269) miss events.

How — Dart-only, no Kotlin / Swift / C++ changes

• The update* handlers lower-case on ingestion, so every event, callback and per-device map key is lower-case. This also lets Match device ids case-insensitively across event streams #269's dual-case stream matching collapse to a single lower-case compare.
• Native BLE calls still need the platform's case (Android's getRemoteDevice requires upper-case and throws otherwise; Apple's peripheral cache is keyed by the upper-case uuidString, Windows parses the address either way, Linux's BlueZ address is upper-case), so the platform implementations convert back at their boundary: a single _nativeId() helper at the pigeon-channel native-op sites, and one line in the Linux instance's device lookup (the single point every Linux op funnels through).

Note the native side receives the same upper-case id it always has — it's just reconstructed at the Dart boundary now instead of being supplied by the caller — so native behaviour is unchanged.

Breaking

Callers that stored or compared an emitted id by exact case (e.g. an Android upper-case MAC) must now lower-case it, or compare case-insensitively. CHANGELOG updated under "next major".

Testing

• 97 Dart tests pass (flutter test) and flutter analyze is clean — including 5 new tests asserting lower-case emission from every update* handler, with the existing Match device ids case-insensitively across event streams #269 case-insensitive-matching / dedup / cache tests still green.
• Verified on a real Android device. I pointed a consumer app (a BLE pet-tracker gateway that round-trips scanned ids straight back into connect()) at this branch and confirmed the whole path: scan → connect() → auth → characteristic streaming → and the reconnect/self-heal path. That's exactly the round-trip this change relies on — Android's getRemoteDevice throws on a lower-case MAC, so a successful connect proves the boundary conversion is doing its job.

Developed with AI assistance (noted via the commit's Co-Authored-By trailer).