frame: fix UDP datagram boundary recovery after parse errors

[gavrilaf]

Problem

When using EndpointUDPClient over a real UDP link (e.g. a radio-link tunnel), a single malformed or out-of-order datagram produces dozens of spurious parse error invalid magic byte errors:

2026/04/11 15:55:46 parse error invalid magic byte: 0
2026/04/11 15:55:46 parse error invalid magic byte: ff
2026/04/11 15:55:46 parse error invalid magic byte: 2b

The root cause: bufio.Reader treats the UDP connection as a byte stream. After a parse error it continues scanning byte-by-byte inside the same datagram, generating one error per remaining byte. The next datagram then starts mid-buffer, corrupting further frames.

This does not affect TCP or serial endpoints because those are true byte streams where byte-by-byte scanning is the correct recovery strategy.

Solution

Add EndpointUDPClientPacket — identical to EndpointUDPClient but marks the channel as packet-oriented. After any ReadError, DiscardBuffered() drops all bytes still buffered from the current datagram so the next Read() begins on a fresh UDP packet boundary.

One bad datagram → exactly one EventParseError → normal parsing resumes.

Changes

• pkg/frame: add Reader.DiscardBuffered() to flush the bufio buffer
• pkg/frame: ReadWriter allocates a 65536-byte buffer in packet-oriented mode so one bufio fill always covers exactly one datagram
• pkg/frame/v2_frame: unmarshal header with Peek + Discard instead of peekAndDiscard so an incompatibility-flag error leaves header bytes in the buffer for DiscardBuffered to clean up
• channel: call DiscardBuffered() on ReadError when PacketOriented
• channel_provider: detect packetOrientedEndpoint interface and set the flag
• endpoint_udp_client_packet: new EndpointUDPClientPacket endpoint type

Tests

• TestEndpointUDPClientPacketRecoverAfterMalformedDatagrams: sends 2 junk datagrams followed by a valid one — verifies exactly 2 EventParseError events and correct frame received after
• TestEndpointUDPClientRecoverAfterMalformedDatagram: existing-style test in endpoint_client_test.go
• TestReaderRecoverAfterInvalidV2IncompatibilityFlag: unit test for the Peek+Discard header change