types: unify all epoch timestamps to i64

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

schema/memory.capnp

@@ -10,7 +10,7 @@
 struct ContentNode {
   uuid       @0  :Data;         # 16 bytes, random
   version    @1  :UInt32;       # monotonic per UUID, latest wins
-  timestamp  @2  :Float64;      # unix epoch
+  timestamp  @2  :Int64;        # unix epoch seconds
   nodeType   @3  :NodeType;
   provenance @4  :Provenance;
   key        @5  :Text;         # "identity.md#boundaries" human-readable
@@ -29,11 +29,15 @@ struct ContentNode {
   stateTag   @16 :Text;         # cognitive state (warm/open, bright/alert, etc.)
 
   # Spaced repetition
-  lastReplayed              @17 :Float64;   # unix epoch
+  lastReplayed              @17 :Int64;     # unix epoch seconds
   spacedRepetitionInterval  @18 :UInt32;    # days: 1, 3, 7, 14, 30
 
   # Section ordering within a file
   position  @19 :UInt32;    # 0 = file-level, 1+ = section index
+
+  # Stable creation timestamp (unix epoch seconds). Set once when the
+  # node is first created; never changes on rename or content update.
+  createdAt @20 :Int64;
 }
 
 enum NodeType {
@@ -62,7 +66,7 @@ enum Category {
 struct Relation {
   uuid       @0 :Data;         # 16 bytes, random
   version    @1 :UInt32;
-  timestamp  @2 :Float64;      # unix epoch
+  timestamp  @2 :Int64;        # unix epoch seconds
   source     @3 :Data;         # content node UUID
   target     @4 :Data;         # content node UUID
   relType    @5 :RelationType;