fix: decode .sqlplan files the way SSMS writes them; name the parse failure

- lib/plan-file.ts: BOM-aware decode for UTF-8, UTF-16LE/BE (with and without BOM) and UTF-32, with a NUL-density fallback for BOM-less UTF-16. The browser had a two-byte FF FE sniff only, so a UTF-16BE or BOM-less plan arrived as NUL-filled junk and the digest blamed the export.
- digestPlan now quotes fast-xml-parser's first line instead of a fixed sentence, so a truncated file, a stray entity and a wrong file stay distinguishable.
- plan-file.ts holds the decode so the XML parser stays out of the client bundle.
- tests: 6 encodings round-trip through decodePlan into a digest; a truncated plan's message is asserted to carry a reason; e2e drops a real UTF-16LE file. 36 unit, 23 e2e.
- history.spec picked the rail's first row, which is now one of the new failing-input rows; it selects the newest finished run instead.

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

lib/digest.ts

@@ -36,7 +36,13 @@ export function digestPlan(raw: string): Digest {
   if (xml.length > MAX_XML_BYTES)
     throw new PlanError(`Plan is ${Math.round(xml.length / 1e6)} MB, over the ${MAX_XML_BYTES / 1e6} MB cap.`);
   let doc: any;
-  try { doc = parser.parse(xml); } catch { throw new PlanError("That XML will not parse — re-export the actual execution plan."); }
+  // Name the parser's complaint. "re-export the plan" is what a user gets told either way, so a truncated
+  // file, a stray entity and a genuinely wrong file have to stay distinguishable.
+  try { doc = parser.parse(xml); }
+  catch (e: any) {
+    const why = String(e?.message ?? e).split("\n")[0].slice(0, 140);
+    throw new PlanError(`The XML stopped parsing: ${why}. Check the file was saved completely (SSMS: Save As, not copy-paste).`);
+  }
   if (!doc?.ShowPlanXML) throw new PlanError("Not a SQL Server ShowPlan XML document. In SSMS: press Ctrl+M, run the query, then save the Execution Plan tab as .sqlplan.");
 
   const stmts = collect(doc, "StmtSimple");