docs: record the AIS unavailable-code handling

CONTRIBUTING asks for docs/CURRENT-STATE.md and CHANGELOG.md in the same PR
as a runtime behaviour change.

Co-Authored-By: Claude Opus 5 (1M context) <[email protected]>

CHANGELOG.md

@@ -1,5 +1,9 @@
 # Changelog
 
+- Report AIS speed and course that carry the standard "not available" code as
+  unknown instead of 102.3 knots and 360 degrees. Genuine readings, including a
+  stopped vessel's zero and the highest encodable values, are unchanged.
+
 - Distinguish PARTIAL vessel snapshots from STALE data in the layer panel, with
   accepted-record counts and unchanged retention, freshness and outage safeguards.
 

docs/CURRENT-STATE.md

@@ -1,5 +1,13 @@
 # God's Eye View Current State
 
+AIS encodes speed over ground in 0.1-knot units and course over ground in
+0.1-degree units, reserving the top code of each field for "not available", so
+those reports arrive as 102.3 knots and 360 degrees. Both are stored as unknown,
+and vessel cards, the HUD and analyst queries show a missing measurement rather
+than a reading. Heading keeps its existing separate check for its own sentinel.
+Genuine readings are unchanged, including a stopped vessel's zero and the
+highest speed and course the fields can encode.
+
 Vessel snapshot completeness is separate from freshness. A current snapshot with
 rejected or duplicate records shows PARTIAL with accepted/received counts; stale
 or unknown freshness and transport failures retain their warnings. Partial