sctp: implement RFC 6525 stream reset for DataChannel close

[nus]

Closes the "DataChannel close doesn't send SCTP stream reset (RECONFIG)" BACKLOG item (RFC 6525 / RFC 8831 §6.7).

Problem

DataChannel.close() flipped local state only — it never sent an OUTGOING_SSN_RESET_REQUEST, the SCTP layer dropped incoming RE-CONFIG chunks (declared but unparsed), and the W3C closing state + onclosing event were missing. Result: closing a channel never told the peer, and a remote-initiated close was silently ignored, so neither side fired onclose.

Change

• chunk.dart: RE-CONFIG chunk (0x82) + Outgoing/Incoming SSN Reset Request and Re-config Response parameters, with encode/parse. TLV framing is shared with the existing INIT-parameter path via _encodeTlv/_concatBytes.
• state_machine.dart: resetStreams() sends an Outgoing SSN Reset Request (one in flight at a time, retransmit timer bounded by Max.Retrans); incoming requests reset the inbound stream, respond, and reset our own outbound side too (loop-guarded) so the channel closes both ways; responses finalize the reset. Streams reported once via a new onStreamReset callback; duplicate requests resend the cached response.
• Interop fix: advertise RE-CONFIG support in INIT/INIT-ACK via the Supported Extensions Parameter (0x8008). Without this Chrome's dcSCTP rejects close() with UNSUPPORTED_OPERATION ("peer doesn't support it") — a gap the live-Chrome e2e caught that loopback tests could not.
• data_channel.dart: real closing state + onClosing event; close() initiates the reset and defers closed/onClose to _finalizeClose() when the reset completes (immediate close when there's no association).
• peer_connection.dart: wire close → resetStreams and onStreamReset → _finalizeClose; PeerConnection.close() finalizes channels immediately.

Tests

• Unit (reconfig_test.dart): chunk encode/parse round-trips, webdartc↔webdartc loopback reset (both peers observe the reset), duplicate-request handling; DataChannel close-state transitions (data_channel_test.dart).
• Live Chrome e2e (e2e_test.dart Scenario 1): webdartc closes the channel and both peers fire onclose (webdartc's onClose + Chrome's dcClosed).

dart analyze clean; full suite passes (635 unit + 22 e2e). Reviewed with /simplify.

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