atperson persistence layer modularisation

Following the v6 persistence feature (#72) and a dead/duplicate logic cleanup pass, modularise the persistence layer to satisfy the atperson naming convention and eliminate duplicated section-walk scaffolding.

Changes

• New atoms: persistence/sections.{c,h} (shared section codecs), persistence/v5.{c,h} (legacy format), persistence/v6.{c,h} (explicit architecture format), persistence/save.c (public save dispatch)
• Reader helpers: atp_reader_next_section and atp_load_finish in reader.c shared by both loaders
• Deleted: encode_common.c, encode_v6.c, encode.c, decode_common.c, decode_v6.c, decode.c — all violated the "no underscored file names, ever" rule
• Required-section masks: Fixed bit positions (bit N-1 for tag N): v5 = 0x3F (tags 1–6), v6 = 0x43F (+ ARCH bit 10)

Verification

All three CI configs (network ON/OFF, ASan) 49/49 green.

Commit: 71a3605