Phase 1a: Rename/split types — DiscoveredPeripheral snapshot + Peripheral handle skeleton

[itsniper]

Parent: #50

Requirements

• Introduce DiscoveredPeripheral as the scan snapshot (current snapshot fields: id, cbIdentifier, name, rssi, lastSeen, advertisement).
• Redefine Peripheral as the long-lived handle type (interned by id); implementation may be class or id-façade (open at planning).
• Update scan streams to emit DiscoveredPeripheral.
• Update Demo + tests + DocC for the rename/split.
• No live CB in public API.

Acceptance criteria

• Snapshot type is DiscoveredPeripheral; handle type is Peripheral
• Existing discovery/list call sites compile and behave
• Breaking rename called out in PR (pre-1.0 OK per PRD)