add position field to nodes for stable section ordering

Sections within a file have a natural order that matters —
identity.md reads as a narrative, not an alphabetical index.

The position field (u32) tracks section index within the file.
Set during init and import from parse order. Export and
load-context sort by position instead of key, preserving the
author's intended structure.

src/migrate.rs

@@ -227,6 +227,7 @@ pub fn migrate() -> Result<(), String> {
             state_tag,
             last_replayed: 0.0,
             spaced_repetition_interval: 1,
+            position: 0,
             community_id: None,
             clustering_coefficient: None,
             schema_fit: None,
@@ -266,6 +267,7 @@ pub fn migrate() -> Result<(), String> {
             state_tag: unit.state.clone().unwrap_or_default(),
             last_replayed: 0.0,
             spaced_repetition_interval: 1,
+            position: 0,
             community_id: None,
             clustering_coefficient: None,
             schema_fit: None,