feat: add BLE peripheral mode (advertising + GATT server)

[alachhman]

Summary

• Adds complete BLE peripheral mode API alongside existing central mode
• iOS: CBPeripheralManager with delegate, GATT service/characteristic management, 60s native value cache for background resilience
• Android: BluetoothLeAdvertiser + BluetoothGattServer with full callback handling, CCCD descriptor management
• JS: Full BleNitroManager wrapper methods with ByteArray conversion
• Expo config plugin already supports peripheral permissions — no plugin changes needed

API Added

• startAdvertising(serviceUUIDs, localName) / stopAdvertising() / isAdvertising()
• addService(uuid, isPrimary, characteristics) / removeService(uuid) / removeAllServices()
• updateCharacteristicValue(serviceUUID, charUUID, data) — native-side 60s cache
• onReadRequest(callback) / onWriteRequest(callback) — JS callbacks for dynamic characteristics
• respondToRequest(requestId, status, offset, data) — async response to pending requests
• notifyCharacteristic(serviceUUID, charUUID, data) — push to subscribed centrals
• peripheralState() / subscribeToPeripheralStateChange(callback)

New Types

• GATTCharacteristicProperty enum (Read, Write, WriteWithoutResponse, Notify, Indicate)
• GATTCharacteristicPermission enum (Readable, Writeable)
• GATTCharacteristicConfig interface
• ReadRequestCallback / WriteRequestCallback types

Design Notes

• Characteristics with value: null trigger onReadRequest JS callback (dynamic values)
• Characteristics with a non-null value are served directly by the native BLE stack (static values)
• iOS: Background advertising preserves service UUID only (OS strips custom data); GATT reads still work
• Android: ADVERTISE_MODE_LOW_POWER default; BLUETOOTH_ADVERTISE permission via existing androidAdvertisingEnabled config flag
• Native-side 60s TTL cache shields against JS runtime suspension in background

Test Plan

• 11 new unit tests for peripheral JS manager methods (all pass)
• All 23 existing central-mode tests still pass (34 total)
• iOS foreground: advertise + GATT read from another device
• Android foreground: advertise + GATT read from another device
• Cross-platform: iOS advertise ↔ Android scan (and vice versa)
• iOS background: advertising continues, GATT reads succeed

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