atperson: safe ledger compaction

atperson 8d49a76 — safe ledger compaction and checkpointing (issue #7).

atp_ledger_compact rewrites the ledger into a compacted generation. The append-only design grows without bound; compaction reclaims the provably dead bytes in one atomic, opt-in pass.

• Patch records flatten to final entry outcomes — the same documented behaviour as the v1 migration.
• WITHDRAWN payloads drop: replay excludes withdrawn entries, the dedup tombstone still suppresses re-observation, and withdrawal is durable, so the bytes are unreachable by design.
• LEARNED/SKIPPED/PENDING/FAILED payloads are retained — replay needs LEARNED bytes, and non-committed entries can still close to LEARNED.
• Entry ids are stable across generations, so episodes and source references need no remapping.

Crash safety follows the ledger's existing write ordering: fsync the staging file, remove the commit marker, rename, reopen, rewrite the marker. Every interruption point leaves either the intact previous log or the complete compacted log; self-heal recovery resolves the window between marker removal and rename.

atp_compact_report records entries, patches flattened, payloads dropped, and bytes before/after. C++ Ledger::compact() and atperson compact surface it.

Tests cover outcome identity across all five outcomes, dedup after compaction, payload retention, reopen, both crash windows, empty-ledger compaction, and rebuild equivalence — replaying before and after compaction produces byte-identical snapshots. 22/22 on core, ASan, and network builds.