Add BLE pairing-mode flag and stale bond recovery

[PaulDWhite]

Summary

• Adds manufacturer data to BLE advertisements (both legacy and extended advertising paths) with a pairing-mode flag byte so the Flutter app can filter non-pairable controllers from the scan list
• During pairing mode, proactively requests fresh security negotiation on connect to recover from stale iOS bonds (rc=19 failures)
• On auth failure during pairing mode, deletes stale peer data and re-initiates pairing instead of immediately rejecting the connection

Details

Manufacturer data format: Espressif company ID (0x02E5 LE) + 1 flag byte

• 0x01 = pairing mode active (controller accepting new bonds)
• 0x00 = normal mode (only bonded devices accepted)

Stale bond recovery: When iOS caches old encryption keys after a controller bond clear, the connection fails with rc=19 (BLE_ERR_UNSPECIFIED). The firmware now:

1. On connect during pairing mode: calls ble_gap_security_initiate() to request fresh security
2. On auth failure during pairing mode: calls NimBLEDevice::deleteBond() for the peer and re-initiates pairing

Test plan

• Flash firmware, verify controller advertises normally with bonded devices
• Enter pairing mode (10s button hold), verify new device can discover and pair
• Clear controller bonds, verify stale iOS bond recovery works during pairing mode
• Verify non-pairing-mode controllers are not connectable by unbonded devices

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR enhances the BLE pairing UX by advertising an explicit “pairing mode” flag for the Flutter app to filter scan results, and by adding recovery behavior for stale iOS bonds during pairing mode.

Changes:

• Add manufacturer data payload (Espressif company ID + pairing-mode flag byte) to both extended and legacy advertising paths.
• During pairing mode, proactively initiate a security exchange on connect to recover from stale iOS bonding state.
• On authentication failure during pairing mode, delete peer bond data and re-initiate pairing instead of immediately disconnecting.

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