feat(exn): migrate packet interfaces to CCSDSPack v2

[Inczert]

Summary

Migrate the EXN mission packet/interface contract to CCSDSPack v2 and make the ICD, runtime .cfg templates, JSON mirrors, and MCU header definitions agree on one wire profile.

Protocol baseline

• CCSDS Space Packet version 0
• CCSDSPack v2.x, validated against v2.0.0
• PUS revision A for all currently defined EXN services
• PUS:revA:TC with one-octet source ID
• PUS:revA:TM with zero-octet destination ID
• CRC-16/CCITT-FALSE packet error control
• TC APID identifies destination endpoint
• TM APID identifies producing endpoint
• mission CUC timestamps remain Application Data

This deliberately removes the old assumption that Service 5 implies a PUS-B secondary header or Service 17 implies PUS-C.

Routing / System HK

• GS device-directed operations use the four-octet proxy preamble on the GS -> MCU hop.
• MCU-local operations do not use the proxy preamble.
• System HK TC 3/10 is direct GS -> MCU, APID 0x100, GS Source ID 0x10.
• Its Application Data is exactly {transactionId:u16, include_mask:u8, detailMask:u16}.
• System HK TM 3/100 is produced by MCU with APID 0x100.

Repository changes

• rewrite the master ICD around the v2 packet profile;
• align GS, MCU, PI and FPGA documentation;
• migrate all current CCSDSPack .cfg templates to v2 selectors/keys;
• add canonical interfaces/mcu-rtos/exn_interfaces.h;
• retain astrai_interfaces.h as a compatibility shim;
• update the current JSON mirrors and document that the JSON set is partial;
• add a CCSDSPack v2 interface validator and GitHub Actions gate.

Validation

Interface CI on the branch validates:

• CCSDSPack v2.0.0 builds and installs cleanly;
• every committed .cfg loads and serializes through ccsds::Packet;
• JSON mirrors parse;
• canonical header compiles as C;
• compatibility header compiles as C++.

Latest branch CI: PASS.

Related

This is the interface/ICD counterpart to the EXN-GS CCSDSPack v2 router/client migration.