proof(xml-7-44): rename prefix field → nsPrefix (0.8.0 keyword clash)

`Xml744.Document.Namespace.prefix` collided with Idris2 0.8.0's
`prefix` reserved word for operator-section syntax, parsing as
"Expected end of input" at the field declaration. Renamed the field
to `nsPrefix` and updated the one consumer (`renderNs`).

Pure rename, no semantic change. With this in place, `Xml744.Document`
compiles (5/6) and `Xml744.Core` compiles (6/6) — the whole toolkit is
green for the first time since the rescue extraction in commit
`0c78bfce`. All 12 exports across the two files check `is total` and
no `assert_total` / `assert_smaller` / `believe_me` / `%unsafe` /
`PROOF_TODO` / `idris_crash` / `partial ` markers remain in any of the
six toolkit modules.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

idris2-ecosystem/xml-toolkit/src/Xml744/Document.idr

@@ -34,7 +34,7 @@ renderDecl d =
 public export
 record Namespace where
   constructor MkNs
-  prefix : String
+  nsPrefix : String
   uri    : String
 
 ||| Common OOXML namespaces
@@ -53,7 +53,7 @@ nsMarkupCompat = MkNs "mc" "http://schemas.openxmlformats.org/markup-compatibili
 ||| Render namespace as attribute
 export
 renderNs : Namespace -> String
-renderNs ns = "xmlns:" ++ ns.prefix ++ "=\"" ++ ns.uri ++ "\""
+renderNs ns = "xmlns:" ++ ns.nsPrefix ++ "=\"" ++ ns.uri ++ "\""
 
 ||| Full XML document
 public export