document selection-bounded change detection and export provenance

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/KNOWN_LIMITATIONS.md

@@ -53,6 +53,15 @@ runtime source of truth.
 - Flood-hazard percentages are shares of the modelled area. Where the VMM model
   does not cover the selection, that is reported as missing coverage and never
   as an absence of risk.
+- Change detection compares whole objects and does not clip them to the
+  selection, so a footprint crossing the selection edge is compared in full.
+  That is deliberate: clipping one snapshot and not the other would report the
+  selection edge itself as a change. The count of such objects is reported.
+- The change-detection GeoJSON is capped by `preview_limit` (changes first).
+  The counts always describe the whole selection; the map does not.
+- An export states its own completeness in `geointel_provenance`. A capped
+  export is still a valid, usable file — it simply no longer implies it holds
+  everything the selection contains.
 
 ## Historical analysis