Source-route header codec (Part 6 §11).

Wire layout (big-endian throughout):

    0       1       2       3
    +-------+-------+-------+-------+
    | ver   | total | curr  | dead-
    +-------+-------+-------+-------+
    ...    deadline (8 bytes)    ...
    +-------+-------+-------+-------+
    ...    path_hash (16 bytes)  ...
    +-------+-------+-------+-------+
    ...    hops[0..total_hops-1] (each 16 bytes — NodeId prefix)
  

• version (1 byte) — currently 1.
• total_hops (1 byte) — 1..8.
• current_hop (1 byte) — index of the hop currently processing. new/2,3 creates a header with 0; each hop advances by 1 before forwarding; the destination observes current_hop = total_hops - 1 and after one final advance sees is_complete/1 = true.
• deadline (8 bytes, unsigned) — absolute Unix ms past which the CALL is dropped (BOLT#4 expiry_too_soon).
• path_hash (16 bytes) — first 16 bytes of SHA-256(concat(hops)). Computed once at the origin and checked at every hop. Tampering with the hop sequence breaks the hash.
• hops — total_hops × 16 bytes; each entry is the first 16 bytes of the NodeId at that hop.

Fixed overhead is 27 bytes (1 + 1 + 1 + 8 + 16). A path of N hops occupies 27 + 16*N bytes; maximum (N = 8) is 155 bytes.

Reference: plans/PLAN_MACULA_V2_PART6_PROTOCOL.md §11; plans/PLAN_MACULA_V2_PART3_DISCOVERY.md §6.6; plans/PLAN_PHASE_4_BREAKDOWN.md Session 4.2.