All epoch timestamp fields (timestamp, last_replayed, created_at on
nodes; timestamp on relations) are now i64. Previously a mix of f64
and i64 which caused type seams and required unnecessary casts.
- Kill now_epoch() -> f64 and now_epoch_i64(), replace with single
now_epoch() -> i64
- All formatting functions take i64
- new_node() sets created_at automatically
- journal-ts-migrate handles all nodes, with valid_range check to
detect garbage from f64->i64 bit reinterpretation
- capnp schema: Float64 -> Int64 for all timestamp fields
Position was only in the bincode cache (serde field) — it would
be lost on cache rebuild from capnp logs. Now persisted in the
append-only log via ContentNode.position @19.
Also fixes journal-tail sorting to extract dates from content
headers, falling back to key-embedded dates.
