fix(labs): keep a reading's conversion record unless its value actually changes

The single-reading edit cleared sourceValue and sourceUnit whenever the body
carried a value, and the web edit sheet always sends one, so a date or note
edit erased the "converted from" hint. It is now cleared only when the value
differs from the stored one. The sheet also opened a converted reading at its
stored precision (100.90191999999999); it now starts from the value as the lab
surfaces show it and leaves an untouched value out of the request, so the
stored full-precision number is not replaced by its rounded display.

prisma/schema.prisma

@@ -6152,7 +6152,7 @@ model LabResult {
   sourceReferenceText String?   @map("source_reference_text")
   /// @pending-consumer(#1095) v1.42 — the value as printed on the source
   /// report, kept when a write converted it into the biomarker's canonical
-  /// unit. NULL when no conversion happened. A later edit of `value` clears it.
+  /// unit. NULL when no conversion happened. An edit that changes `value` clears it.
   sourceValue         Float?    @map("source_value")
   /// @pending-consumer(#1095) v1.42 — the unit the source report printed,
   /// paired with `sourceValue`.