atperson — deterministic e2e lifecycle harness

Added tests/e2e_harness.cpp (issue #28): a deterministic end-to-end scenario harness that exercises the real durable pipeline — ledger files, graph snapshots, ingestion state — without network access. The only fixture is the page fetcher; everything else is the production code path.

Scenarios:

• Multi-run catch-up — three scripted pages, one page per run, two restarts from disk; asserts resumed cursors, accumulated checkpoints, and the final snapshot.
• Crash recovery — a run dies between ledger commits and the snapshot save; the ledger is authoritative, and a replay rebuild recovers the full learned state.
• Transient failure — a transport throw leaves the persisted cursor and snapshot untouched; retry resumes from the checkpoint.
• Duplicate suppression — a replayed timeline is fully deduplicated by the ledger across restarts.
• Withdrawal flows — deleted-post and blocked-author withdrawals, rebuild, and a pinned invariant: WITHDRAWN is a committed outcome, so a withdrawn observation reappearing in the feed stays a duplicate and is never re-learned.
• Planning abstention — empty and unknown contexts abstain with the documented reasons on recovered state; learned contexts decide deterministically.
• Cross-run determinism — equivalent scenario runs produce byte-identical final learned state.

Both build configurations (network off/on) pass 33/33 tests. Commit 1eecf4e.