proof(xml-7-44): rename Namespace.prefix → nsPrefix (Idris2 0.8.0 keyword) (#82)

## Summary

One-line rename to clear the final baseline-rot blocker in the
idris2-xml toolkit. `Xml744.Document.Namespace.prefix` collides with
Idris2 0.8.0's `prefix` reserved word for operator-section syntax — the
field declaration parses as "Expected end of input". Renamed the field
to `nsPrefix` and updated the one consumer (`renderNs`).

Pure rename, no semantic change.

## Result: toolkit fully green

With #79 + #80 + #81 + this branch, every module builds:

```
1/6: Building Xml744.Escape
2/6: Building Xml744.Text
3/6: Building Xml744.Attribute
4/6: Building Xml744.Element
5/6: Building Xml744.Document
6/6: Building Xml744.Core
```

First clean build since the rescue extraction in commit `0c78bfce`.

12 exports across `Document` + `Core` check `is total`. Across the full
toolkit:

```
$ grep -nE 'assert_total|assert_smaller|believe_me|%unsafe|PROOF_TODO|idris_crash|partial ' src/Xml744/*.idr | grep -v 'without `assert'
(no matches — only doc-comment mentions describing what is NOT used)
```

Total damage to land all six green: 4 PRs touching 4 files; 9 proof
debts retired; 40 exported names verified `is total`; zero proof escapes
remain.

## Stack

Stacks on top of #81 → #80 → #79. Base is `main` per the orphan-trap
guidance, so the diff is cumulative until the earlier PRs land, then
this rebases to a 4-line single-file change.

## Test plan

- [ ] CI green
- [ ] `idris2 --build idris2-ecosystem/xml-toolkit/xml-7-44.ipkg`
reports 6/6 modules built (verified locally)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

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