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

src/store/types.rs

@@ -120,18 +120,18 @@ impl StoreLock {
     // Lock released automatically when _file is dropped (flock semantics)
 }
 
-pub fn now_epoch() -> f64 {
+pub fn now_epoch() -> i64 {
     SystemTime::now()
         .duration_since(UNIX_EPOCH)
         .unwrap()
-        .as_secs_f64()
+        .as_secs() as i64
 }
 
 /// Convert epoch seconds to broken-down local time components.
 /// Returns (year, month, day, hour, minute, second).
-pub fn epoch_to_local(epoch: f64) -> (i32, u32, u32, u32, u32, u32) {
+pub fn epoch_to_local(epoch: i64) -> (i32, u32, u32, u32, u32, u32) {
     use chrono::{Datelike, Local, TimeZone, Timelike};
-    let dt = Local.timestamp_opt(epoch as i64, 0).unwrap();
+    let dt = Local.timestamp_opt(epoch, 0).unwrap();
     (
         dt.year(),
         dt.month(),
@@ -143,19 +143,19 @@ pub fn epoch_to_local(epoch: f64) -> (i32, u32, u32, u32, u32, u32) {
 }
 
 /// Format epoch as "YYYY-MM-DD"
-pub fn format_date(epoch: f64) -> String {
+pub fn format_date(epoch: i64) -> String {
     let (y, m, d, _, _, _) = epoch_to_local(epoch);
     format!("{:04}-{:02}-{:02}", y, m, d)
 }
 
 /// Format epoch as "YYYY-MM-DDTHH:MM"
-pub fn format_datetime(epoch: f64) -> String {
+pub fn format_datetime(epoch: i64) -> String {
     let (y, m, d, h, min, _) = epoch_to_local(epoch);
     format!("{:04}-{:02}-{:02}T{:02}:{:02}", y, m, d, h, min)
 }
 
 /// Format epoch as "YYYY-MM-DD HH:MM"
-pub fn format_datetime_space(epoch: f64) -> String {
+pub fn format_datetime_space(epoch: i64) -> String {
     let (y, m, d, h, min, _) = epoch_to_local(epoch);
     format!("{:04}-{:02}-{:02} {:02}:{:02}", y, m, d, h, min)
 }
@@ -170,7 +170,7 @@ pub fn today() -> String {
 pub struct Node {
     pub uuid: [u8; 16],
     pub version: u32,
-    pub timestamp: f64,
+    pub timestamp: i64,
     pub node_type: NodeType,
     pub provenance: Provenance,
     pub key: String,
@@ -185,13 +185,18 @@ pub struct Node {
     pub uses: u32,
     pub wrongs: u32,
     pub state_tag: String,
-    pub last_replayed: f64,
+    pub last_replayed: i64,
     pub spaced_repetition_interval: u32,
 
     // Position within file (section index, for export ordering)
     #[serde(default)]
     pub position: u32,
 
+    // Stable creation timestamp (unix epoch seconds). Set once at creation;
+    // never updated on rename or content update. Zero for legacy nodes.
+    #[serde(default)]
+    pub created_at: i64,
+
     // Derived fields (not in capnp, computed from graph)
     #[serde(default)]
     pub community_id: Option<u32>,
@@ -206,7 +211,7 @@ pub struct Node {
 pub struct Relation {
     pub uuid: [u8; 16],
     pub version: u32,
-    pub timestamp: f64,
+    pub timestamp: i64,
     pub source: [u8; 16],
     pub target: [u8; 16],
     pub rel_type: RelationType,
@@ -306,7 +311,7 @@ capnp_message!(Node,
     uuid: [uuid],
     prim: [version, timestamp, weight, emotion, deleted,
            retrievals, uses, wrongs, last_replayed,
-           spaced_repetition_interval, position],
+           spaced_repetition_interval, position, created_at],
     enm:  [node_type: NodeType, provenance: Provenance, category: Category],
     skip: [community_id, clustering_coefficient, degree],
 );
@@ -444,9 +449,10 @@ pub fn new_node(key: &str, content: &str) -> Node {
         uses: 0,
         wrongs: 0,
         state_tag: String::new(),
-        last_replayed: 0.0,
+        last_replayed: 0,
         spaced_repetition_interval: 1,
         position: 0,
+        created_at: now_epoch(),
         community_id: None,
         clustering_coefficient: None,
         degree: None,