ODS writer: drop duplicate XML declaration; harden sheet-name sanitiser.

LibreOffice rejected the generated `.ods` with `Format error at 2,39
in content.xml`. Root cause: `pugi::format_no_declaration` suppresses
only the *implicit* declaration auto-added at save time — the explicit
`node_declaration` I had appended to the document still got serialised,
on top of a manual `<?xml…?>` string prepend in the output. Two
declarations back-to-back, invalid XML.

Fix: let pugixml emit the explicit declaration node, drop the manual
prepend.

Also harden the sheet-name sanitiser in the export action: ODS / Excel
also forbid `< > &` in raw cell or table names, so the default
connection name `bp/J20 <-> payload1/P0` made content.xml entity-
escape `<` to `&lt;`, which a few viewers handle but Excel rejects.
Clip to 31 chars too (Excel's hard limit) so multi-name connections
don't blow up the open.

Verified by `soffice --headless --convert-to csv` round-tripping the
output without errors.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/imports/ods_writer.cpp

@@ -73,11 +73,14 @@ std::string build_content_xml(const std::vector<OdsSheet> &sheets) {
     }
 
     std::ostringstream oss;
-    doc.save(oss, "  ", pugi::format_no_declaration | pugi::format_raw);
-    // pugi's `format_no_declaration` drops the `<?xml…?>` line; we want
-    // it. Re-emit explicitly to control encoding.
-    return std::string(R"(<?xml version="1.0" encoding="UTF-8"?>)" "\n")
-           + oss.str();
+    // We added the declaration node explicitly above, so pugi will emit
+    // it. `format_no_declaration` only suppresses *implicit* injection;
+    // setting it here would leave the explicit node in place and silently
+    // duplicate would-be auto-emission elsewhere. Keep format_raw to
+    // avoid pretty-print overhead — the file is read by software, not
+    // humans.
+    doc.save(oss, "", pugi::format_raw);
+    return oss.str();
 }
 
 // Minimum META-INF/manifest.xml that satisfies the spec.

src/tui/commands_export.cpp

@@ -98,12 +98,18 @@ void Tui::RegisterExportCommands() {
                     int total = 0;
                     for (auto &ckv : *sys->connections()) {
                         Connection *c = ckv.second;
-                        // Sheet names: Excel rejects /\?*:[] characters
-                        // (LibreOffice tolerates them but stays portable).
+                        // Sheet names: Excel rejects /\?*:[] characters,
+                        // ODS forbids < > & in raw cell/table names
+                        // (they'd need entity-escaping but a few viewers
+                        // choke on them). Sanitise to underscores.
                         std::string sname = c->name;
                         for (char &ch : sname)
                             if (ch == '/' || ch == '\\' || ch == '?' || ch == '*'
-                                || ch == ':' || ch == '[' || ch == ']') ch = '_';
+                                || ch == ':' || ch == '[' || ch == ']'
+                                || ch == '<' || ch == '>' || ch == '&') ch = '_';
+                        // Sheet names also can't exceed 31 chars in
+                        // Excel; clip aggressively to stay portable.
+                        if (sname.size() > 31) sname = sname.substr(0, 31);
                         OdsSheet *s = w.add_sheet(sname);
                         const char *hdr[] = {
                             "transform",